Fix README formatting
Make MIT license explicit
Document poll_timer_setup
Align includes in main.commented to those of main.c
Refer to `hw_interface.c` instead of `KEYBOARD_MODEL.c`
Align matrix poll code in main.commented to that of main.c

The code in the commented main file probed the matrix row by row while
the current main code probes the matrix column by column. This commit
changes the commented code to probe in the same way the main code does.

This commit fixes also a small typo in the description of the return
value of `probe_column` (it returns `0b00000000` for non-pressed keys,
not `0b00000001`).
Remove trailing whitespace
Remove NOTES file

The `NOTES` file contains only test matrices already present in
`model/common.h`.
Add .gitignore file
Merge pull request #6 from gioele/documentation

Documentation fixes
Move KEYBOARD_MODEL include to implementation
Remove redundant function declarations
Use strbool instead of uint8_t
Raise error if there is no suitable impl of update_leds()
Use model dates in include guards