Add doc about mbtiles diff command

docs/src/mbtiles-diff.md

@@ -0,0 +1,13 @@
+# Diffing MBTiles
+
+## `mbtiles diff`
+
+Diff command compares two mbtiles files `file A` and `file B`, and generates a delta (diff) file. If the diff file is [applied](mbtiles-copy.md#mbtiles-apply-patch) to `A`, it will produce `B`.  
+The delta file will contain all tiles that are different between the two files (modifications, insertions, and deletions as `NULL` values), for both the tile and metadata tables.  
+The only exception is `agg_tiles_has` metadata value. It will be renamed to `agg_tiles_hash_in_diff` and a new `agg_tiles_hash` will be generated for the diff file itself.
+
+```shell
+# This command will comapre `file_a.mbtiles` and `file_b.mbtiles`, and generate a new diff file `diff_result.mbtiles`,This command will compares file_a.mbtiles and file_b.mbtiles, and generates a new diff file diff_result.mbtiles 
+# If the diff file is applied to file_a, it will produce file_b. 
+mbtiles diff file_a.mbtiles file_b.mbtiles diff_result.mbtiles // 
+```