Add nobrowserify flag.
:memo: Updates README.md.

- Adds info about `nobrowserify`.
Create new nobrowserify attribute.

- Useful for bower_components (example paper elements).
- Useful for old libraries with no npm / common.js etc.
:memo: Updating README.md.
Update README.md
Merge pull request #20 from Custom-Elements/test_nobrowserify_flag

Test nobrowserify flag