contrib: (asmap) add docs about encode and decode commands

Author: jurraca

contrib/asmap/README.md

@@ -15,6 +15,31 @@ These commands may take a few minutes to run with `python3`,
 depending on the amount of data involved and your machine specs.
 Consider using `pypy3` for a faster run time.
 
+### Encoding and Decoding
+
+ASmap files are somewhat large in text form, and need to be encoded
+to binary before being used with Bitcoin Core.
+
+The `encode` command takes an ASmap and an output file.
+
+The `--fill`/`-f` flag further reduces the size of the output file
+by assuming an AS assignment for an unmapped network if an adjacent network is assigned.
+This procedure is lossy, in the sense that it loses information
+about which ranges were unassigned.
+However, if the input ASmap is incomplete,
+this procedure will also reassign ranges that should have an AS assignment,
+resulting in an ASmap that may diverge from reality significantly.
+Finally, another consequence is that the resulting encoded file
+will no longer be meaningful for diffs.
+Therefore only use `--fill` if
+you want to optimise space, you have a reasonably complete ASmap,
+and do not intend to diff the file at a later time.
+
+The `decode` command takes an encoded ASmap and an output file.
+As with `encode`, the `--fill`/`-f` flag reduces the output file size
+by reassigning subnets. Conversely, the `--non-overlapping`/`-n` flag
+increases output size by outputting strictly non-overlapping network ranges.
+
 ### Comparing ASmaps
 
 AS control of IP networks changes frequently, therefore it can be useful to get