Bug 1587650 - Update README.md with more informative intro r=gw
Bug 1587650 - Update README.md with more informative intro r=gw

Addresses issues with the README in [mozilla bug 1587650](https://bugzilla.mozilla.org/show_bug.cgi?id=1587650).
[import_pr] From https://github.com/servo/webrender/pull/3772

Differential Revision: https://phabricator.services.mozilla.com/D49501

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/fc99c73779480bdc4f82b2cb6e91530cddd48f40

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender