add Mac environment setup in README

[tinio]

No description provided.

[phillipsj]

I can help with this one also if needed.

[phillipsj]

I added some basic instructions for a mac. I somehow got confused and merged it with the master. Hope that was okay.

[tinio]

@phillipsj no worries about merging to master for this. Again, thanks for contribution.

[tinio]

@mtlee are you still having issues getting set up on the mac?

If it's a problem with pysqlite, just try installing from my clone of the library:
https://github.com/tinio/pysqlite
pip install git+git://github.com/tinio/pysqlite.git@extension-enabled#egg=pysqlite

[tinio]

moving to week 4 milestone