Improve the `Support` section from `README.md`
Improve inline comments

* Fix typos.
* Update and improve the information.
* Improve consistency (phrasing, grouping etc.).
* Add more links to the official Apache documentation.
Add info on mod_pagespeed & content transformation

Provide information about `mod_pagespeed` not rewriting any/some
of the resources if the `Cache-Control: no-transform` response header
is set.

Ref: https://developers.google.com/speed/pagespeed/module/configuration#notransform

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Thanks to @Nikita-S-Doroshenko for pointing this out!

Close h5bp/server-configs-apache#46