Rename embassy-hal-common to embassy-hal-internal, document it's for internal use only. (#1700)

embassy-hal-internal/README.md

@@ -0,0 +1,16 @@
+# embassy-macros
+
+An [Embassy](https://embassy.dev) project.
+
+Internal implementation details for Embassy HALs. DO NOT USE DIRECTLY. Embassy HALs (`embassy-nrf`, `embassy-stm32`, `embassy-rp`) already reexport
+everything you need to use them effectively.
+
+## License
+
+This work is licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  <http://www.apache.org/licenses/LICENSE-2.0>)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)
+
+at your option.