Add installation method from git to README.md

[jasikpark]

Would it be helpful to add instructions to help people install the "bleeding edge" of jetpack? I think maybe the subtitle for the section that I added could be edited to encourage only the people who need the latest github version from downloading via github, but I would have found this helpful when I wanted jetpack global update to work.

[jasikpark]

It could also be reworded from:

Or, install from the latest on Github:

to

Or, install from the nightly on Github:

to discourage casual installation via Github

[ankane]

Hey @jasikpark, thanks for another PR 👍 I think it just muddies the installation instructions, so don't think it makes sense to add for now.

[jasikpark]

After reviewing, I agree