Documentation updates for Mbed TLS 3.0

Update documentation to reflect the branch changes.

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

BRANCHES.md

@@ -6,8 +6,12 @@ At any point in time, we have a number of maintained branches consisting of:
   this always contains the latest release, including all publicly available
   security fixes.
 - The [`development`](https://github.com/ARMmbed/mbedtls/tree/development) branch:
-  this is where new features land,
-  as well as bug fixes and security fixes.
+  this is where the next major version of Mbed TLS (version 3.0) is being
+  prepared. It has API changes that make it incompatible with Mbed TLS 2.x,
+  as well as all the new features and bug fixes and security fixes.
+- The [`development_2.x`](https://github.com/ARMmbed/mbedtls/tree/development_2.x) branch:
+  this branch retains the API of Mbed TLS 2.x, and has a subset of the
+  features added after Mbed TLS 2.26.0 and bug fixes and security fixes.
 - One or more long-time support (LTS) branches:
   these only get bug fixes and security fixes.
 
@@ -48,6 +52,7 @@ The following branches are currently maintained:
 
 - [master](https://github.com/ARMmbed/mbedtls/tree/master)
 - [`development`](https://github.com/ARMmbed/mbedtls/)
+- [`development_2.x`](https://github.com/ARMmbed/mbedtls/tree/development_2.x)
 - [`mbedtls-2.16`](https://github.com/ARMmbed/mbedtls/tree/mbedtls-2.16)
  maintained until at least the end of 2021, see
   <https://tls.mbed.org/tech-updates/blog/announcing-lts-branch-mbedtls-2.16>

ChangeLog.d/mbedtls3.0.txt

@@ -0,0 +1,3 @@
+Changes
+   * There is ongoing work for the next release (= Mbed TLS 3.0.0 branch to
+     be released 2021-xx-xx), including various API-breaking changes.

README.md

@@ -5,6 +5,11 @@ Mbed TLS is a C library that implements cryptographic primitives, X.509 certific
 
 Mbed TLS includes a reference implementation of the [PSA Cryptography API](#psa-cryptography-api). This is currently a preview for evaluation purposes only.
 
+Stability
+---------
+
+**Warning: the [`development`](https://github.com/ARMmbed/mbedtls/tree/development) branch of Mbed TLS currently has an unstable API.** It is where work is happening on the next major release of Mbed TLS. Until Mbed TLS 3.0 is released, if you need a stable API, please use the branch [`development_2.x`](https://github.com/ARMmbed/mbedtls/tree/development_2.x) instead.
+
 Configuration
 -------------