Help: Add missing release note for XCODE_EMBED_*

In 5651901c54 (Xcode: add support for embedding frameworks, 2020-10-24)
we neglected to add a release note for the new target properties
that were added.

Help/release/dev/xcode-embed-frameworks.rst

@@ -0,0 +1,9 @@
+xcode-embed-frameworks
+----------------------
+
+* When using the Xcode generator, it is now possible to embed frameworks
+  using the new :prop_tgt:`XCODE_EMBED_FRAMEWORKS <XCODE_EMBED_<type>>`
+  target property.  Aspects of the embedding can be customized with the
+  :prop_tgt:`XCODE_EMBED_FRAMEWORKS_PATH <XCODE_EMBED_<type>>`,
+  :prop_tgt:`XCODE_EMBED_FRAMEWORKS_CODE_SIGN_ON_COPY` and
+  :prop_tgt:`XCODE_EMBED_FRAMEWORKS_REMOVE_HEADERS_ON_COPY` target properties.