Add component documentation for the HTML5 parser.

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+# Updating HTML5 parser code
+
+Our html5 parser is based on the java html5 parser from [Validator.nu](http://about.validator.nu/htmlparser/) by Henri Sivonen. It has been adopted by Mozilla and further updated, and has been imported as a whole into the UXP tree to have an independent and maintainable source of it that doesn't rely on external sources.
+
+## Stages
+Updating the parser code consists of 3 stages:
+- Making updates to the html parser source in java
+- Let the java parser regenerate part of its own code after the change
+- Translate the java source to C++
+
+This process was best explained in the [following Bugzilla comment](https://bugzilla.mozilla.org/show_bug.cgi?id=1378079#c6), which explain how to add a new attribute name to html5, inserted in this document for convenience:
+
+>> Is
+>> there any documentation on how to add a new nsHtml5AttributeName?
+>
+> I don't recall. I should get around to writing it.
+>
+>> Looks like
+>> I need to clone hg.mozilla.org/projects/htmlparser/ and generate a hash with
+>> it?
+>
+> Yes. Here's how:
+>
+> `cd parser/html/java/`
+> `make sync`
+>
+> Now you have a clone of [https://hg.mozilla.org/projects/htmlparser/](https://hg.mozilla.org/projects/htmlparser/) in > parser/html/java/htmlparser/
+>
+> `cd htmlparser/src/`
+> `$EDITOR nu/validator/htmlparser/impl/AttributeName.java`
+>
+> Search for the word "uncomment" and uncomment stuff according to the two comments that talk about uncommenting
+> Duplicate the declaration a normal attribute (nothings special in SVG mode, etc.). Let's use "alt", since it's the first one.
+> In the duplicate, replace ALT with IS and "alt" with "is".
+> Search for "ALT,", duplicate that line and change the duplicate to say "IS,"
+> Save.
+>
+> `javac nu/validator/htmlparser/impl/AttributeName.java`
+> `java nu.validator.htmlparser.impl.AttributeName`
+>
+> Copy and paste the output into nu/validator/htmlparser/impl/AttributeName.java replacing the text below the comment "START GENERATED CODE" and above the very last "}".
+> Recomment the bits that you uncommented earlier.
+> Save.
+>
+> `cd ../..` - Back to parser/html/java/
+> `make translate`
+
+## Organizing commits
+
+**The html5 parser code is fragile due to its generation and translation before being used as C++ in our tree. Do not touch or commit anything without a code peer nearby with knowledge of the parser and the commit process (at this moment that means Gaming4JC (@g4jc)), and communicate the changes thoroughly.**
+
+To organize this properly in our repo, commits should be split up when making these kinds of changes:
+1. Commit your code edits to the html parser
+2. Regenerate java into a translation-ready source
+3. Commit
+4. Translate and regenerate C++ code
+5. Check a build to make sure the changes have the intended result
+6. Commit
+
+This is needed because the source edit will sometimes be in parts that are self-generated and may otherwise be lost in generation noise, and because we want to keep a strict separation between commits resulting from developer work and those resulting from running scripts/automated processes.
+
+
+