[docs] Add documentation for common/

Part of #17913.

Author: zcorpan

common/README.md

@@ -0,0 +1,12 @@
+The files in this directory are non-infrastructure support files that can be used by tests.
+
+* `blank.html` - An empty HTML document.
+* `css-red.txt` - A text/plain "CSS" document.
+* `domain-setter.sub.html` - An HTML document that sets `document.domain`.
+* `dummy.xhtml` - An XHTML document.
+* `dummy.xml` - An XML document.
+* `entities.json` - All named character references in HTML.
+* `text-plain.txt` - A text/plain document.
+* `*.js` - Utility scripts. These are documented in the source.
+* `*.py` - wptserve [Python Handlers](https://web-platform-tests.org/writing-tests/python-handlers/). These are documented in the source.
+* `security-features` - Documented in `security-features/README.md`.

common/security-features/README.md

@@ -33,7 +33,7 @@ except that:
   always `top`, which represents the top-level generated test HTML.
   (This entry is omitted in the JSON passed to JavaScript, but
   the policy deliveries specified here are written as e.g.
-  <meta> elements in the generated test HTML or HTTP headers)
+  `<meta>` elements in the generated test HTML or HTTP headers)
 - Instead of `PolicyDelivery` object (in `sourceContextList` or
   `subresourcePolicyDeliveries`), following placeholder strings can be used.