contrib: Add a README file with instructions.

The instructions are pretty straight forward.

contrib/README.md

@@ -0,0 +1,47 @@
+# Contributed code to RTEMS
+
+If it is located within this directory it is *not* maintained by the RTEMS
+project.
+
+## Updating
+
+To update code in this directory follow these steps:
+
+1. Update code to the latest upstream version as a pure import.
+2. Create a tag for that import.
+3. Apply any changes required on top as separate commit(s).
+
+Using this we will be able to diff between upstream versions and our own
+changes.
+
+## Commit message
+
+In the import commit message add:
+
+1. Upstream hash if pulling from a repository
+2. Upstream version if release. If development version add hash.
+3. URL file was taken from or cloned from.
+
+## Tag format
+
+The tag should always include the full path in the contrib directory with the
+version appended.
+
+If there is no version or it is a hash use `<dev version>-<hash>`.
+
+For example:
+
+- contrib/bsps/arm/stm32u5/stm32u5xx-hal-driver/1.5.0
+
+The path is `contrib/bsps/arm/stm32u5/stm32u5xx-hal-driver` with `1.5.0` being the version.
+
+## Current Sources
+
+These are the current source directories and where they are from. Look at the
+Git log in order to find out version and update details
+
+| Directory                             | Source                                                         |
+| ------------------------------------- | -------------------------------------------------------------- |
+| bsps/arm/stm32h7/cmsis-device-h7/     | https://github.com/STMicroelectronics/cmsis_device_h7.git      |
+| bsps/arm/stm32h7/stm32h7xx_hal_driver | https://github.com/STMicroelectronics/stm32u5xx-hal-driver     |
+| bsps/arm/stm32u5/stm32u5xx-hal-driver | https://github.com/STMicroelectronics/stm32h7xx_hal_driver.git |