docs: describe remote signing upgrade process

This commit adds a short guide that explains how a remote signing node
setup should be migrated from lnd v0.14.x-beta to lnd v0.15.x-beta and
adds a note to all 0.15.x release notes.

docs/release-notes/release-notes-0.15.2.md

@@ -14,15 +14,19 @@
 * [A bug has been fixed that caused fee estimation to be incorrect for taproot
   inputs when using the `SendOutputs` call.](https://github.com/lightningnetwork/lnd/pull/6941)
 
-# Contributors (Alphabetical Order)
+## Taproot
 
 
+**NOTE** for users running a remote signing setup: A manual account import is
+necessary when upgrading from `lnd v0.14.x-beta` to `lnd v0.15.x-beta`, see [the
+remote signing documentation for more
+details](../remote-signing.md#migrating-a-remote-signing-setup-from-014x-to-015x).
+
 ## Performance improvements
 
 * [Refactor hop hint selection
   algorithm](https://github.com/lightningnetwork/lnd/pull/6914)
 
-
 # Contributors (Alphabetical Order)
 
 * Eugene Siegel