added README

README.md

+# Briar Mailbox
+
+This project aims to develop an easy way for Briar users to increase their 
+reachability and lower the battery drain of their phone.
+
+In Briar messages are exchanged directly between contacts (peer-to-peer). 
+This kind of synchronous message exchange requires contacts to be online and 
+connected to each other.  
+While this is great for privacy (no central server which
+can log things or be censored) it's bad for reachability, especially in
+mobile networks where connectivity can be limited.
+
+```mermaid
+graph LR
+ A[Alice]
+ B[Bob]
+ A1[Alice]
+ B1[Bob]
+ style B fill:#8db600
+ style A1 fill:#8db600
+ subgraph Alice offline
+ B-. can't send message .-> A
+ end
+ subgraph Bob offline
+ B1-. can't send message .-> A1
+ end
+```
+
+Message delivery could be delayed for an arbitrary time (or even indefinitely) 
+until both Bob and Alice are online at the same time.  
+The repeater solves this problem by providing 
+mailbox-like message buffer where contacts can leave messages for the owner 
+of the repeater and which is connected to a stable internet connection 
+(e.g. the wifi at home, cable internet) and a power source.
+ 
+
+```mermaid
+graph LR
+  A[Alice]
+  A1[Alice]
+  B[Bob]
+  B1[Bob]
+  RA["Mailbox (always online)"]
+  style B fill:#8db600
+  style RA fill:#8db600
+  style A1 fill:#8db600
+  subgraph Alice offline
+  B-. can't send message .-> A
+  end
+  subgraph Alices' Mailbox
+  B-- send message --> RA
+  end
+  subgraph Alice online
+  B1-. can't send message .-> A1
+  A1-- get message --> RA
+  end
+```
+
+## Hardware
+
+We want the repeater to be as easy to deploy as possible. The first version 
+will come as Android application since it will be easy to setup and besides a 
+spare phone no special hardware is required. Once this is done support for
+any hardware supporting Java (e.g. unix server, raspberry pi) will be added.
+
+## Features
+
+### Core features
+
+* Allow contacts to store messages for the owner of the repeater
+* Allow the owner to store messages for her contacts. Contacts can pick them up
+  when syncing with the repeater.
+* Owner and contacts connect to the repeater via Tor.
+
+### Extended features/components
+
+* The repeater can sync group messages (from groups the owner is part of) with 
+  other group members (increases message circulation)
+* Contacts and the owner can connect to the repeater via other transports (Bluetooth, Wifi-Direct, Lan)
+* Push-like message notification for the owner to decrease battery drain
+
+
+

[Torsten Grote]

@avon maybe clarify that these are not actually implemented (yet) as this confuses people.