[sksl] Document atomics and barriers in README

* Added documentation for the new synchronization primitives to the
  SkSL README.
* Renamed sksl/README to sksl/README.md. This allows the page to get
  rendered as markdown when viewed in the browser.

Bug: b/260597257

Change-Id: I4b98ea460820d955c603e354db1918b05c6f289c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/618519
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
2 files changed