docs(messages): Add a msg/json/README.md (#6635)

* docs(messages): Add a msg/json/README.md

Add a README.md file to msg/json/ explaining the purpose of the
files therein, mentioning that PRs are not accepted but that
translation updates should be made via TranslateWiki, and with a
short description of how to build the langfiles that are generated
from them.

* docs(messages): Add note about the Klingon exception

msg/json/README.md

@@ -0,0 +1,33 @@
+# Blockly
+
+## Messages
+
+This directory contains "messages" files, which are JSON-format files
+containing human-translated strings that are needed by Blockly.
+
+### Translating
+
+**We do not accept pull requests for files in this directory**.
+Instead, we use [Translatewiki](https://translatewiki.net/) to manage
+translations for Blockly.  Please refer to [the detailed instructions
+on how to help us translate Blockly](
+https://developers.google.com/blockly/guides/contribute/core/translating)
+on the Blockly Developers site.
+
+There is one notable exception: because the language is not supported
+by Translatewiki, contributors to the [Klingon translation](
+https://developers.google.com/blockly/guides/contribute/core/klingon)
+may submit PRs making changes to [`tlh.json`](tlh.json) directly.
+
+### Building
+
+Blockly cannot use the files in this directory directly.  Instead,
+they must first be converted into executable `.js` files we call
+"langfiles".  This is done automatically as needed by the build
+system, but you can also run this build step manually by invoking `npm
+run langfiles`; the output will be found in `build/msg/`.
+
+## Additional Information
+
+For more information about Blockly, see [the main Blockly README](
+../../README.md).