Update README.md to indicate GLES 3.1 conformance This CL updates the README.md to indicate that ANGLE became OpenGL ES 3.1 conformant on July, 2020. Bug: angleproject:3520 Change-Id: I5e93560ab7927a81b55bd6ed89abb5d78b704993 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2289949 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Tim Van Patten <timvp@google.com>