README.md: Fix/Update the "Git usage" section
Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/README.md b/README.md
index d745b24..33ad4ac 100644
--- a/README.md
+++ b/README.md
@@ -46,7 +46,15 @@
### Git usage
-The `development` branch and the `mbedtls-3.6` long-term support branch of Mbed TLS use a [Git submodule](https://git-scm.com/book/en/v2/Git-Tools-Submodules#_cloning_submodules) ([framework](https://github.com/Mbed-TLS/mbedtls-framework)). This is not needed to merely compile the library at a release tag. This is not needed to consume a release archive (zip or tar).
+The supported branches (see [`BRANCHES.md`](BRANCHES.md)) use [Git submodules](https://git-scm.com/book/en/v2/Git-Tools-Submodules#_cloning_submodules). They contain two submodules: the [framework](https://github.com/Mbed-TLS/mbedtls-framework) submodule and the [tf-psa-crypto](https://github.com/Mbed-TLS/TF-PSA-Crypto) submodule, except for the 3.6 LTS branch, which contains only the framework submodule. Release tags also use Git submodules.
+
+After cloning or checking out a branch or tag, run:
+ ```
+ git submodule update --init --recursive
+ ```
+ to initialize and update the submodules before building.
+
+However, the official source release tarballs (e.g. [mbedtls-4.0.0-beta.tar.bz2](https://github.com/Mbed-TLS/mbedtls/releases/download/mbedtls-4.0.0-beta/mbedtls-4.0.0-beta.tar.bz2)) include the contents of the submodules.
### Generated source files in the development branch