encoding/README.md: add links

Add links to code locations to the README.md for modules/encoding. Also
clarify that only ISO-2022-JP has been patched in ICU.

Change-Id: Id1058d8ab4971da05c59e7f65b4f2f69987e7419
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397973
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805145}

third_party/blink/renderer/modules/encoding/README.md

@@ -4,6 +4,8 @@ This directory contains Blink's implementation of [the Encoding
 Standard API](https://encoding.spec.whatwg.org/#api).
 
 Actual encoding and decoding is delegated to implementations of the
-WTF::TextCodec interface. For most encodings we then delegate to ICU. The
-version of ICU used by Blink has been patched for better conformance with the
-standard.
+[WTF::TextCodec](../../platform/wtf/text/text_codec.h) interface in
+[platform/wtf/text](../../platform/wtf/text). For most encodings we then
+delegate to [ICU](../../../../icu). The [ISO-2022-JP
+encoding](https://encoding.spec.whatwg.org/#iso-2022-jp) implementation of ICU
+has been patched for better conformance with the Encoding Standard.