Improve documentation on writing wpt-server test

[izgzhen]

wpt-server is used to support more forms of testings, like testing the correctness of form-submission request.

There are some tests utilizing this feature such as tests/wpt/web-platform-tests/html/semantics/forms/form-submission-0/submit-entity-body.html.

But in our documentation https://github.com/servo/servo/blob/master/tests/wpt/README.md (or even in http://testthewebforward.org), there is little guide or example for it.

[nox]

http://testthewebforward.org seems to have documentation now.

@jdm Can we close this?

[jdm]

I don't see the wpt-server documentation linked anywhere from testthewebforward.org, and our own documentation doesn't link to it either.