Create README.md

Copied and pasted from the old documentation for `--experimental-specifier-resolution`

Author: dfabulich

commonjs-extension-resolution-loader/README.md

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+Node's ESM specifier resolution does not support all default behavior of
+the CommonJS loader. One of the behavior differences is automatic resolution
+of file extensions and the ability to import directories that have an index
+file.
+
+Use this loader to enable automatic extension resolution and importing from
+directories that include an index file.
+
+```console
+$ node index.mjs
+success!
+$ node index # Failure!
+Error: Cannot find module
+$ node --loader=./loader.js index
+success!
+```