Add release note for SkRP.

Change-Id: I5b492b040971ca2d8d3c640235e96996d18739b6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/679656
Commit-Queue: John Stiles <[email protected]>
Commit-Queue: Chris Mumford <[email protected]>
Reviewed-by: Chris Mumford <[email protected]>
Auto-Submit: John Stiles <[email protected]>

Author: johnstiles-google

relnotes/README.md

@@ -24,5 +24,8 @@ followed.
 2. Do not use [headings](https://www.markdownguide.org/basic-syntax/#headings).
    Milestones in the top level release notes file (RELEASE_NOTES.md) use a
    single heading per milestone.
-3. Horizontal rules will automatically be inserted between milestones when the
+3. Do not use start your note with an
+   [asterisk](https://www.markdownguide.org/basic-syntax/#unordered-lists) or
+   other leading marks. These are automatically inserted by the script.
+4. Horizontal rules will automatically be inserted between milestones when the
    release notes are generated

relnotes/skrp.md

@@ -0,0 +1,2 @@
+The CPU backend for Runtime Effects has been rewritten. This may cause slight differences in
+performance and image quality when runtime effects are painted onto a raster surface.