misc: Add KCONFIG.md file which talks about Kconfig files.
misc: Add KCONFIG.md file which talks about Kconfig files.

There are some minor aspects about how Kconfig files are used in gem5
which are a little different from how they are documented online or are
used in other projects. Document that in this file.

Change-Id: Ib981d1e265cf466d453071b33820d6baa24e17d4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58634
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5