add readme section

Author: rajveermalviya

gen/README.md

@@ -2,6 +2,18 @@
 
 The generator for generating `webgpu.h` header and other extension headers or implementation specific headers from the yaml spec files.
 
+# Generate implementation specific header
+
+The generator also allows generating custom implementation specific headers that build on top of `webgpu.h` header. The generator accepts the combination of `-yaml` & `-header` flags in sequence, which it uses to validate the specifications and then generate their headers.
+
+For example, if `wgpu.yml` contains the implementation specific API, the header can be generated using:
+
+```shell
+> go run ./gen -schema schema.json -yaml webgpu.yml -header webgpu.h -yaml wgpu.yml -header wgpu.h
+```
+
+Since the generator does some duplication validation, the order of the files matter, so generator mandates the core `webgpu.yml` to be first in the sequence.
+
 # yaml spec
 
 ### Types