Add a tutorial on how to use git-cms-merge-topic.
Merge pull request #243 from ktf/gh-pages

Add a tutorial on how to use git-cms-merge-topic.
Add tutorial on how to merge usercode and CMSSW.
Merge pull request #247 from ktf/gh-pages

Add tutorial on how to merge usercode and CMSSW.
Add -- to make sure it always works.
Updated with more git diff examples.
Merge pull request #251 from ktf/gh-pages

Improve cvs diff equivalents.