Document building the gem from master.

[ci skip]

README.md

@@ -524,3 +524,20 @@ your changes merged back into core is as follows:
     $ rake gemspec
     $ gem build gollum.gemspec
     $ gem push gollum-X.Y.Z.gem
+    
+## BUILDING THE GEM FROM MASTER
+
+```
+$ git clone https://github.com/github/gollum.git
+$ cd gollum
+gollum$ rake build
+Updated gollum.gemspec
+mkdir -p pkg
+gem build gollum.gemspec
+  Successfully built RubyGem
+  Name: gollum
+  Version: 2.1.0
+  File: gollum-2.1.0.gem
+mv gollum-2.1.0.gem pkg
+gollum$ gem install pkg/gollum-2.1.0.gem
+```