doc/go1.15: add release notes for plugin

Add a blurb to the release notes mentioning that the
linker now supports DWARF generation for -buildmode=plugin,
and that plugin builds work now for freebsd/amd64.

Updates #37419.

Change-Id: I84da7a52af84a9d765f73ca7ea525e7af8d64f05
Reviewed-on: https://go-review.googlesource.com/c/go/+/236162
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: thanm

Diff for: doc/go1.15.html

@@ -486,6 +486,19 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl>
 
+<dl id="plugin"><dt><a href="/pkg/plugin/">plugin</a></dt>
+  <dd>
+    <p><!-- CL 182959 -->
+      DWARF generation is now supported (and enabled by default) for <code>-buildmode=plugin</code> on macOS.
+    </p>
+  </dd>
+  <dd>
+    <p><!-- CL 191617 -->
+      Building with <code>-buildmode=plugin</code> is now supported on <code>freebsd/amd64</code>.
+    </p>
+  </dd>
+</dl>
+
 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
   <dd>
     <p><!-- CL 228902 -->