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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Abhra303

Documentation/technical/bitmap-format.txt

@@ -125,6 +125,10 @@ MIDXs, both the bit-cache and rev-cache extensions are required.
 
 		** The compressed bitmap itself, see Appendix A.
 
+	* TRAILER:
+
+		Index checksum of the above contents. It is a 20-byte SHA1 checksum.
+
 == Appendix A: Serialization format for an EWAH bitmap
 
 Ewah bitmaps are serialized in the same protocol as the JAVAEWAH