Update BRANCHES

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

Author: daverodgman

BRANCHES.md

@@ -6,12 +6,13 @@ 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 current major version of Mbed TLS (version 3.x) is being
-  prepared. It has API changes that make it incompatible with Mbed TLS 2.x,
+  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.
 - One or more long-time support (LTS) branches: these only get bug fixes and
-  security fixes. Currently, the only supported LTS branch is:
-  [`mbedtls-2.28`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-2.28).
+  security fixes. Currently, the supported LTS branches are:
+- [`mbedtls-2.28`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-2.28).
+- [`mbedtls-3.6`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-3.6).
 
 We retain a number of historical branches, whose names are prefixed by `archive/`,
 such as [`archive/mbedtls-2.7`](https://github.com/Mbed-TLS/mbedtls/tree/archive/mbedtls-2.7).
@@ -23,10 +24,10 @@ the API of 3.(x+1) is backward compatible with 3.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.
 
-Every major version will become an LTS branch when the next major version is
-released. We may occasionally create LTS branches from other releases at our
-discretion.
-When a new LTS branch is created, it usually remains supported for three years.
+We will make regular LTS releases on an 18-month cycle, each of which will have
+a 3 year support lifetime. On this basis, 3.6 LTS (released March 2024) will be
+supported until March 2027. The next LTS release will be a 4.x release, which is
+planned for September 2025.
 
 ## Backwards Compatibility for application code
 
@@ -104,8 +105,11 @@ The following branches are currently maintained:
 
 - [master](https://github.com/Mbed-TLS/mbedtls/tree/master)
 - [`development`](https://github.com/Mbed-TLS/mbedtls/)
+- [`mbedtls-3.6`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-3.6)
+ maintained until March 2027, see
+  <https://github.com/Mbed-TLS/mbedtls/releases/tag/v3.6.0>.
 - [`mbedtls-2.28`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-2.28)
- maintained until at least the end of 2024, see
-  <https://github.com/Mbed-TLS/mbedtls/releases/tag/v2.28.7>.
+ maintained until the end of 2024, see
+  <https://github.com/Mbed-TLS/mbedtls/releases/tag/v2.28.8>.
 
 Users are urged to always use the latest version of a maintained branch.