Add readme documenting how to use the server

README.md

+# XMPP Prekey Server for OTRv4 and forward
+
+This project implements an XMPP Component (XEP-114) that allows XMPP server
+providers to deploy an OTRv4 Prekey Server
+(https://github.com/otrv4/otrv4-prekey-server) in an easy way.
+
+In order to use this component, you also need to run the Raw prekey server (https://github.com/otrv4/otrng-prekey-server/tree/master/server/raw).
+
+If you're using Prosody, you can add configuration options like these to add the server:
+
+```
+component_ports = { 42335 }
+component_interface = "127.0.0.1"
+
+Component "prekeys.example.com"
+        component_secret = "changeme"
+        name = "OTR Prekey Server"
+```
+
+You also need to add a DNS s2s entry for the domain you're using. 
+
+For ejabberd you can add an equivalent ejabberd_service.
+
+Assuming you have the raw server running on localhost with the default port you
+can now start the xmpp server like this:
+
+```
+$ ./prekey-server-xmpp -xmpp-address 127.0.0.1 -xmpp-port 42335 -shared-secret "changeme" -name prekeys.example.com -fingerprint 123AAAAAAAAAAAAAAAAA123CDCDCDABABABABABA88797676556456421132425673535467575765AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
+```
+
+This component will now be discoverable as prekeys.example.com inside the XMPP
+server - it will respond to disco#info and disco#items, and messages sent to it
+will be forwarded to the Raw prekey server.