Improve FASM documentation to boost adoption of the standard

[mgielda]

FPGA Assembly is a file format designed by the SymbiFlow Project developers to provide a plain text file format for configuring the internals of an FPGA.

It is designed to allow FPGA place and route to not care about the actual bitstream format used on an FPGA.

FASM is meant to enhance standardisation across tools and vendors and so is very important to be properly understood and implemented wherever possible.

For that, FASM needs better documentation, and there is a number of tasks that need to be performed to get there.

See:

• Get readthedocs.io working for this repository chipsalliance/fasm#11

And:

• Write and publish a FASM "feature naming" style guide chipsalliance/fasm#4
• Explore using waxeye instead of textx for parsing chipsalliance/fasm#6
• Add "make format" target for fasm chipsalliance/fasm#8
• Import default Python .gitignore config. chipsalliance/fasm#10
• Create specification for common "FASM annotations" chipsalliance/fasm#13