Add some basic commands about Github.

DONE:
- init, pull, remote add origin, add, status, commit, push

TODO:
- Not sure when to use “git init”, should we use it every time we want to submit sth ?
- Not sure when to use “git remote add origin”. Same question as above.
- Could we use some name other than “origin” ?
- What is the meaning of “-u” in the “git push”command ?
- Need the commands to change the account.