bitmap-format.txt: add information for trailing checksum

Bitmap file has a trailing checksum at the end of the file. However
there is no information in the bitmap-format documentation about it.

Add a trailer section to include the trailing checksum info in the
`Documentation/technical/bitmap-format.txt` file.

Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/bitmap-format.txt

@@ -138,6 +138,10 @@ in the index.
 
 	** The compressed bitmap itself, see Appendix A.
 
+	* {empty}
+	TRAILER: ::
+		Trailing checksum of the preceding contents.
+
 == Appendix A: Serialization format for an EWAH bitmap
 
 Ewah bitmaps are serialized in the same protocol as the JAVAEWAH