Add README.md for modules/encoding

Create a README file for blink/renderer/modules/encoding.

Change-Id: I713876693d0a604d08d68bc0d0e275b5976b4ae0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386683
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803527}

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

+# Encoding API
+
+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.