Documentation updates for Mbed TLS 3.0

Update documentation to reflect the branch changes.

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/BRANCHES.md b/BRANCHES.md
index d514418..dae9f0c 100644
--- a/BRANCHES.md
+++ b/BRANCHES.md
@@ -6,8 +6,10 @@
   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 new features normally land, as well as bug fixes and security
+  fixes.
+- The [`development_2.x`] branch:
+  this is where features and bug-fixes destined for 2.x land.
 - One or more long-time support (LTS) branches:
   these only get bug fixes and security fixes.
 
@@ -48,6 +50,7 @@
 
 - [master](https://github.com/ARMmbed/mbedtls/tree/master)
 - [`development`](https://github.com/ARMmbed/mbedtls/)
+- [`mbedtls-2.x`](https://github.com/ARMmbed/mbedtls/tree/mbedtls-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>
diff --git a/ChangeLog.d/mbedtls3.0.txt b/ChangeLog.d/mbedtls3.0.txt
new file mode 100644
index 0000000..73b7cc0
--- /dev/null
+++ b/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.
diff --git a/README.md b/README.md
index 759ffb5..c9ac3ad 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,13 @@
 
 Mbed TLS includes a reference implementation of the [PSA Cryptography API](#psa-cryptography-api). This is currently a preview for evaluation purposes only.
 
+Stablility
+----------
+
+The [`development_2.x`] branch will contain work for the next Mbed TLS 2.x release, and will retain a stable API.
+
+The [`development`] branch is where work focused on the up-coming Mbed TLS 3.0 release takes place - this means that (in addition to everything in [`development_2.x`]) it will introduce API incompatibilities and is not recommended for users. New features will normally land here. 
+
 Configuration
 -------------