Document code formatter.

@gedare has worked quite hard to provide an excellent RTEMS-style formatting tool using Clang-Format.

Some of this initial work can be seen in rtems/rtos/rtems!238 and rtems/rtos/rtems!237

We need to document:

1. How the tool is used.
   1. @chris has suggested we create some new commands waf format and waf format-check to handle running the tool.
2. What rules we apply when deciding to turn formatting off for a code block.
3. What files the formatter is run on. @joel has suggested we disable it for inline ASM heavy files but maybe we can get away with disabling blocks.
   1. Disabled files should be disabled at the top of the file so there is awareness the files are not auto-formatted and it simplifies that we can just run the tool over all source.

The second point above is mostly what this issue is about.