Added contribution-workflow to docs

docs/contribution-workflow

+Update to latest Version
+========================
+
+    git pull
+
+Show local changes
+==================
+
+    git status
+
+    git diff HEAD
+
+    git diff --stat HEAD^ HEAD 
+
+    git log
+
+Commit local changes
+====================
+
+    git add . 
+
+    git commit -am "Message"
+
+    git push
+
+Revert all local changes
+========================
+
+    git reset --hard
+
+Sources
+=======
+
+* http://www.cheat-sheets.org/saved-copy/git-cheat-sheet.svg
+* https://gitorious.hermann-mayer.net/embedded-home-automation/slave-stack
+