README.md: document transformations using filter

Document how the `--filter' option can be somewhat (ab)used to apply
transformations to tests data.
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

README.md

@@ -47,6 +47,24 @@ $ ./parser.py --filter "x['result'] == 'FAILURE'" ...
 
 Filtering takes place after the configuration rules, which are described below.
 
+This filtering mechanism can also be (ab)used to transform tests results.
+
+Example command, which adds a "comment" field (and keeps all the tests):
+
+``` {.sh}
+$ ./parser.py \
+      --filter "x.update({'comment': 'Transformed'}) or True" ...
+```
+
+Example command, which removes filenames prefixes in the "log" field (and keeps
+all the tests):
+
+``` {.sh}
+$ ./parser.py \
+      --filter "x.update({'log': re.sub(r'/.*/', '', x['log'])}) \
+                or True" ...
+```
+
 ## Configuration file
 
 It is possible to use a configuration file with command line option `--config