Backporting and stable/LTS releases: re-introduce older LTSs section

Just after removing the old section, there has been a Zulip thread [1]
asking for such support again.

Thus re-introduce the original text, slightly tweaked.

Link: https://rust-for-linux.zulipchat.com/#narrow/channel/291565-Help/topic/Seeking.20Advice.20on.20Backporting.20Rust.20Support.20to.20Linux.205.2E10/near/499618538 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

src/Backporting-and-stable-LTS-releases.md

@@ -9,3 +9,17 @@ We do our best to maintain the existing Rust support in Linux v6.1 LTS, v6.6 LTS
   - Linux v6.12 LTS is the first LTS that had a minimum supported Rust version, i.e. unpinned.
 
     We will do our best to avoid having to establish a maximum Rust version for that LTS, i.e. to support future Rust releases. However, given the use of unstable features back then, we cannot guarantee it.
+
+## Older LTS releases
+
+There has been some interest over time in backported Rust support for Linux v5.10 LTS and v5.15 LTS.
+
+In general, maintaining an LTS branch, even if based on an official one, requires substantial effort and a long-term commitment, as it should be consistently supported until reaching its End of Life. If we were to consider it, there would need to be a strong demand and/or additional resources provided. If your company, organization or team would be interested, then please [contact us](Contact.md).
+
+There would be several points to consider:
+
+  - The level of support and the security and scheduling implications.
+
+  - Whether all new abstractions, drivers and overall features appearing in mainline are backported, and whether those that require extra backports on the C side to support them should be included.
+
+  - Whether the Rust version policy would be different than the one in mainline, e.g. whether the Rust version would be fixed (like it is in older upstream LTS kernels).