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.
Get package developer to move onto new working branch ASAP.
Ensure branch point is from devel
Added link to Portuguese translation
Merge pull request #2791 from jiahao/doc-new

Updates package contribution instructions