doc: Add README.md

Add README file for the repo, describing the content. Add docs
for how to use update scripts to import content from Simplicity
SDK.

Signed-off-by: Aksel Skauge Mellbye <[email protected]>

Author: asmellby

README.md

@@ -0,0 +1,46 @@
+# Zephyr HAL for Silicon Labs
+
+This module provides the HAL content needed to build Zephyr applications for Silicon Labs devices.
+
+The following SDKs are imported:
+
+* `gecko` - Gecko SDK for EFM32 and EFR32 Series 0 and 1 devices
+* `si32` - Precision32 HAL for SiM3U devices
+* `simplicity_sdk` - Simplicity SDK for EFM32 and EFR32 Series 2 devices
+* `wiseconnect` - WiSeConnect SDK for SiWx91x devices
+
+## Simplicity SDK
+
+### Updating Simplicity SDK
+
+Updates to Simplicity SDK are performed using the scripts available in the `scripts` directory.
+To import a new version of Simplicity SDK, run
+`./scripts/update_simplicity_sdk.py --src <path/to/simplicity_sdk> --signoff`,
+where `path/to/simplicity_sdk` points to a checked out copy of the Simplicity SDK git repo.
+
+This will revert any Zephyr-specific patches, import the new version of Simplicity SDK, and
+reapply any patches that still apply. Patches that don't apply cleanly are called out by the
+script and skipped, and should be investigated to see if the patch is no longer needed, or if
+it needs to be adapted to changes in the HAL. Patches are discovered by searching the git history
+backwards until the first commit message starting with `simplicity_sdk: Import [...]` is
+encountered.
+
+Zephyr-specific patches to Simplicity SDK must have a commit message that starts with
+`simplicity_sdk: Patch [...]` in order to be automatically identified by the update script.
+
+The script `import_simplicity_sdk.py` does the job of importing the content and updating blobs in
+module.yml, and contains the file list used to filter Simplicity SDK content needed in Zephyr.
+This script is called internally by `update_simplicity_sdk.py`, but can also be run standalone.
+
+If new content is imported from Simplicity SDK, update the file list in one commit, and use the
+prefix `simplicity_sdk: Add [...]` in the commit message of a separate commit that adds the
+additional content. Strive to use the same revision of Simplicity SDK as pre-existing content to
+keep the HAL consistent. If the scripts listed above are used to assist in adding the new content,
+ensure that patches aren't accidentally undone -- the commit should only contain new files, no
+modified files. If patches are needed, add these in separate commits with the
+`simplicity_sdk: Patch [...]` prefix.
+
+### Generating Devicetree Bindings
+
+Python scripts named `scripts/gen_*.py` are used to generate Devicetree bindings for devices
+supported by Simplicity SDK. See the help text of each script for usage details.