Updated ESP32 README.md installation instructions

[AnthonyBriggs]

Some gotchas that I discovered while installing and flashing.

[dpgeorge]

Regarding the bit about pyserial: if you look at the ESP IDF instrutions for Linux at http://esp-idf.readthedocs.io/en/latest/get-started/linux-setup.html it does actually say that you need to install pyserial for the IDF to work. So, instead of adding this extra paragraph, I think it would be more appropriate to make it more explicit that one must follow all the ESP IDF instructions.

[AnthonyBriggs]

Yes, they install it with this line: sudo apt-get install git wget make libncurses-dev flex bison gperf python python-serial.

But if you're running a non-system/apt Python (eg. python 3.6 on debian like I am), then those instructions don't work: the default (2.7) Python will get pyserial instead, and the build will break. Maybe I could include an explicit "If you're running a non-system python then you might see this error ('can't import pyserial') and need to pip install esptools"?

[dpgeorge]

But if you're running a non-system/apt Python (eg. python 3.6 on debian like I am), then those instructions don't work: the default (2.7) Python will get pyserial instead, and the build will break.

Yes, that's true, but in that case you already have a non-standard set up and would then hopefully know how to fix issues like missing Python libraries. Right?

[AnthonyBriggs]

Hopefully. but it also looks possible if you're using apt python 3 (there's also a python3-serial). I just thought it better for the first build and install to be super clear with zero potential blockers if possible.

[dpgeorge]

Thanks for updating the PR. Merged with very minor changes in 7a8be510f52b816a53e05bb30fa5c94638fe225d