LP#1302113: define a file for storing lists of folks to acknowledge in the release notes

If a file called _acknowlegments is present in the docs/RELEASE_NOTES_NEXT
directory, its contents will be appended to the end of the release
notes under an "Acknowledgments" heading when
create_release_notes.sh is run.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Dan Wells <dbw2@calvin.edu>
LP#1302113: acknowledgments for 2.6.0

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Dan Wells <dbw2@calvin.edu>