Add docs and change log entry.

Author: philderbeast

ChangeLog.md

@@ -7,6 +7,8 @@ Release notes:
 * Further to the release notes for Stack 2.3.1, the `-static` suffix has been
   removed from the statically linked Linux/x86_64 binaries.
 
+* Add unpacking of source repository packages (`extra-deps`).
+
 **Changes since v2.11.1:**
 
 Major changes:

doc/unpack_command.md

@@ -17,3 +17,6 @@ By default:
 *   the package is unpacked into a directory named after the package and its
     version. Pass the option `--to <directory>` to specify the destination
     directory.
+
+If PACKAGE is the name of a source repository package or a suffix of its URL
+then the unpacking is direct, not via a package index.