Document BINDGEN_EXTRA_CLANG_ARGS env variable

[tmfink]

Feature was originally introduced in pull-request #1537

[tmfink]

I was hunting for this feature and couldn't find it in the documentation.
I finally found the original pull request via Google, which is less than ideal.

I understand that for most users this is not required (or recommended), but when you need this it's important. I think leaving out the documentation is worse than keeping the documentation.

We should probably also add it to the README (since end consumers of bindgen will probably look there first).

[emilio]

Thank you for doing this. Yeah, adding this to either the book or the README would be appreciated!