Add README file

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+Erebos
+======
+
+The erebos binary provides simple CLI interface to the decentralized Erebos
+messaging service. Local identity is created on the first run.
+
+Erebos identity is based on locally stored cryptographic keys, all
+communication is end-to-end encrypted. Multiple devices can be attached to the
+same identity, after which they function interchangeably, without any one being
+in any way "primary"; messages and other state data are then synchronized
+automatically whenever the devices are able to connect with one another.
+
+Usage
+-----
+
+On the first run, local identity will be created for this device based on
+interactive prompts for:
+
+`Name:` name of the user/owner, which will be shared among all devices
+belonging to the same user; keep empty when initializing device that is going
+to be attached to already existing identity on other device.
+
+`Device:` name describing current device, can be empty.
+
+After the initial setup, the erebos tool presents interactive prompt for
+messages and commands. All commands start with the slash (`/`) character,
+followed by command name and parameters (if any) separated by spaces. When a
+peer or contact is selected, message to send him can be entered directly on the
+command prompt.
+
+### Messaging
+
+`/peers`  
+List peers with direct network connection. Peers are discovered automatically
+on local network or can be manually added.
+
+`/contacts`  
+List known contacts (see below).
+
+`/<number>`  
+Select contact or peer `<number>` based on previous `/contacts` or `/peers`
+output list.
+
+`<message>`  
+Send `<message>` to selected contact.
+
+`/history`  
+Show message history for selected contact or peer.
+
+### Add contacts
+
+To ensure the identity of the contact and prevent man-in-the-middle attack,
+generated verification code needs to be confirmed on both devices to add
+contacts to contact list (similar to bluetooth device pairing). Before adding
+new contact, list peers using `/peers` command and select one with `/<number>`.
+
+`/contacts`  
+List already added contacts.
+
+`/contact-add`  
+Add selected peer as contact. Six-digit verification code will be computed
+based on peer keys, which will be displayed on both devices and needs to be
+checked that both numbers are same. After that it needs to be confirmed using
+`/contact-accept` to finish the process.
+
+`/contact-accept`  
+Confirm that displayed verification codes are same on both devices and add the
+selected peer as contact. The side, which did not initiate the contact adding
+process, needs to select the corresponding peer with `/<number>` command first.
+
+`/contact-reject`  
+Reject contact request or verification code of selected peer.
+
+### Attach other devices
+
+Multiple devices can be attached to single identity to be used by the same
+user. After the attachment process completes the roles of the devices are
+equivalent, both can send and receive messages independently and those
+messages, along with any other sate data, are synchronized automatically
+whenever the devices can connect to each other.
+
+The attachment process and underlying protocol is very similar to the contact
+adding described above, so also generates verification code based on peer keys
+that needs to be checked and confirmed on both devices to avoid potential
+man-in-the-middle attack.
+
+Before attaching device, list peers using `/peers` command and select the
+target device with `/<number>`.
+
+`/attach`  
+Attach current device to the selected peer. After the process completes the
+owner of the selected peer will become owner of this device as well. Six-digit
+verification code will be displayed on both devices and the user needs to check
+that both are the same before confirmation using the `/attach-accept` command.
+
+`/attach-accept`  
+Confirm that displayed verification codes are same on both devices and complete
+the attachment process (or wait for the confirmation on the peer device). The
+side, which did not initiate the attachment process, needs to select the
+corresponding peer with `/<number>` command first.
+
+`/attach-reject`  
+Reject device attachment request or verification code of selected peer.
+
+### Other
+
+`/peer-add <host> [<port>]`  
+Manually add network peer with given hostname or IP address.
+
+`/update-identity`  
+Interactively update current identity information
+
+
+Storage
+-------
+
+Data are by default stored within `.erebos` subdirectory of the current working
+directory. This can be overriden by `EREBOS_DIR` environment variable.
+
+Private keys are currently stored in plaintext under the `keys` subdirectory of
+the erebos directory.
diff --git a/erebos.cabal b/erebos.cabal
index 93051b5..8cd2143 100644
--- a/erebos.cabal
+++ b/erebos.cabal
@@ -14,6 +14,8 @@ Description:
     one being in any way "primary"; messages and other state data are then
     synchronized automatically whenever the devices are able to connect with
     one another.
+    .
+    See README for usage of the CLI tool.
 License:             BSD-3-Clause
 License-File:        LICENSE
 Homepage:            http://erebosprotocol.net
@@ -22,6 +24,8 @@ Maintainer:          roman.smrz@seznam.cz
 Category:            Network
 Stability:           experimental
 Build-type:          Simple
+Extra-Doc-Files:
+    README.md
 Extra-Source-Files:
     src/Erebos/ICE/pjproject.h