Update markdown-gradle-plugin to 1.0.0

and rename documentation links from .html to .md, since
markdown-gradle-plugin will now automatically transfer the links .md to
.html.

Now users can broswe the documentation in their markdown form (e.g. via
github) and via html.

Also add a symlink from README.md to index.md in documentation/.

documentation/connections.md

@@ -1,7 +1,7 @@
 Smack: XMPPConnection Management
 ================================
 
-[Back](index.html)
+[Back](index.md)
 
 Creating a Connection
 ---------------------