Add specification for WebGL 2.0 compute (#2769)

* Add specification for WebGL 2.0 compute

* addressed feedback from Jiajie

* Add a few sub-section to clarify undefined behaviors

* Declared draft status and implementations may not shipped

* Move spec status and copyright from W3C to Khronos

* Addressed feedback from Alexey Knyazev

* Addressed one more comment from Alexey Knyazev

Author: Yunchao He

specs/latest/2.0-compute/Makefile

@@ -0,0 +1,7 @@
+.PHONY: all index.html
+
+all: index.html
+
+index.html: index.bs
+		curl https://api.csswg.org/bikeshed/ -F file=@index.bs -F output=err
+		curl https://api.csswg.org/bikeshed/ -F file=@index.bs -F force=1 > index.html | tee