Convert abarth's "Strings in Blink" doc to a README in the tree

In August 2013, Adam Barth wrote an overview of the WTF::String type[1]
and associated classes. Now that we prefer to keep docs in the tree,
move it into Blink's platform/wtf/text directory as a README.

The following edits were made to the doc:

* Markdown formatting (headers, italics, code)
* Updated StringImpl flag count/descriptions
* Updated method names with new casing/spelling.
* Dropped discussion of String::Append() (removed)
* Dropped #blink IRC channel reference


[1] https://docs.google.com/document/d/1kOCUlJdh2WJMJGDf-WoEQhmnjKLaOYRbiHz5TiGJl14/edit?usp=sharing

Change-Id: I6e6c92ef21d32f647487e0558c3fdf66cd7f7e51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1555003
Auto-Submit: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648730}

third_party/blink/renderer/platform/wtf/text/wtf_string.h

@@ -57,8 +57,7 @@ enum UTF8ConversionMode {
              ? op##IgnoringASCIICase args               \
              : op##IgnoringCase args)
 
-// You can find documentation about this class in this doc:
-// https://docs.google.com/document/d/1kOCUlJdh2WJMJGDf-WoEQhmnjKLaOYRbiHz5TiGJl14/edit?usp=sharing
+// You can find documentation about this class in README.md in this directory.
 class WTF_EXPORT String {
   USING_FAST_MALLOC(String);