Add some instructions to do a release

RELEASE.rdoc

+== Prerequisite 
+
+- Install "git-flow" (brew install git-flow)
+- Install "bump" gem (gem install bump)
+
+== Perform a release
+
+- git flow release start <release>
+- Update the CHANGELOG.rdoc file
+- bump current
+- bump patch
+- rm -rf dist
+- rake spec
+- rake gem
+- git flow release finish <release>
+
+- rake gem:publish