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/.

build.gradle

@@ -6,7 +6,7 @@ buildscript {
         maven { url 'http://dl.bintray.com/content/aalmiray/kordamp' }
     }
     dependencies {
-        classpath 'org.kordamp:markdown-gradle-plugin:0.1.1'
+        classpath 'org.kordamp:markdown-gradle-plugin:1.0.0'
 		classpath 'org.kordamp.gradle:clirr-gradle-plugin:0.1.0'
     }
 }