Merge pull request #10436 from ronald-cron-arm/root-documentation

Update documentation at repo root

Author: gilles-peskine-arm

BRANCHES.md

@@ -6,9 +6,8 @@ At any point in time, we have a number of maintained branches, currently consist
   this always contains the latest release, including all publicly available
   security fixes.
 - The [`development`](https://github.com/Mbed-TLS/mbedtls/tree/development) branch:
-  this is where the next major version of Mbed TLS (version 4.0) is being
-  prepared. It has API changes that make it incompatible with Mbed TLS 3.x,
-  as well as all the new features and bug fixes and security fixes.
+  this is where the next minor version of Mbed TLS 4.x is prepared. It contains
+  new features, bug fixes, and security fixes.
 - One or more long-time support (LTS) branches: these only get bug fixes and
   security fixes. Currently, the supported LTS branches are:
 - [`mbedtls-3.6`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-3.6).
@@ -19,7 +18,7 @@ These branches will not receive any changes or updates.
 
 We use [Semantic Versioning](https://semver.org/). In particular, we maintain
 API compatibility in the `main` branch across minor version changes (e.g.
-the API of 3.(x+1) is backward compatible with 3.x). We only break API
+the API of 4.(x+1) is backward compatible with 4.x). We only break API
 compatibility on major version changes (e.g. from 3.x to 4.0). We also maintain
 ABI compatibility within LTS branches; see the next section for details.
 
@@ -66,25 +65,6 @@ crypto that was found to be weak) may need to be changed. In case security
 comes in conflict with backwards compatibility, we will put security first,
 but always attempt to provide a compatibility option.
 
-## Backward compatibility for the key store
-
-We maintain backward compatibility with previous versions of the
-PSA Crypto persistent storage since Mbed TLS 2.25.0, provided that the
-storage backend (PSA ITS implementation) is configured in a compatible way.
-We intend to maintain this backward compatibility throughout a major version
-of Mbed TLS (for example, all Mbed TLS 3.y versions will be able to read
-keys written under any Mbed TLS 3.x with x <= y).
-
-Mbed TLS 3.x can also read keys written by Mbed TLS 2.25.0 through 2.28.x
-LTS, but future major version upgrades (for example from 2.28.x/3.x to 4.y)
-may require the use of an upgrade tool.
-
-Note that this guarantee does not currently fully extend to drivers, which
-are an experimental feature. We intend to maintain compatibility with the
-basic use of drivers from Mbed TLS 2.28.0 onwards, even if driver APIs
-change. However, for more experimental parts of the driver interface, such
-as the use of driver state, we do not yet guarantee backward compatibility.
-
 ## Long-time support branches
 
 For the LTS branches, additionally we try very hard to also maintain ABI