Updates package contribution instructions

- Adds instructions to migrate development from master to devel branch
- Adds comments for all but the most basic git commands
- Adds caution reminding developers to be familiar with git and seek help when it breaks
- Tells user to delete custom branch after submission to METADATA is accepted.

Minor structural changes and reorganizations, e.g. base doc page now has link to Chinese Julia manual in Chinese.

Follow-up to discussion in julia-users
https://groups.google.com/forum/#!topic/julia-users/J1O3XMp_ybo