Clarification of Platform Support

[johnoneil]

I believe this is similar to #248 in that I'm attempting to use Surfman to support GLES on Linux. According to that issue that is not currently possible without forking the code and adding that support.

My reading of the code suggests this is still the case.

If this is so, I'd suggest clarifying the related README similar to what follows:

The library supports the following platforms:

...

* Windows, with OpenGL ES via Google's ANGLE library.
...

* Android P and up, with OpenGL ES.

[fabricedesre]

Fwiw, I hit the same issue when doing a quick hack to get Servo running on a Pinephone Pro device (see https://fosstodon.org/@capyloon/111032248243803371).

We run under a wayland compositor so by default we are handled a GL api, while the device actually supports GL ES. I'm not sure if there is a way to do better runtime detection.