Document how to minimise corpuses.

Change-Id: Ie487163787d78d867e34709fb34b4c6a836f668d
Reviewed-on: https://boringssl-review.googlesource.com/7275
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/FUZZING.md b/FUZZING.md
index 4bd15a3..16eede8 100644
--- a/FUZZING.md
+++ b/FUZZING.md
@@ -38,3 +38,9 @@
 | `cert`    | 3072            |
 | `server`  | 1024            |
 | `client`  | 4096            |
+
+## Minimising the corpuses
+
+When a large number of new seeds are available, it's a good idea to minimise the corpus so that different seeds that trigger the same code paths can be deduplicated.
+
+In order to minimise all the corpuses, build for fuzzing and run `./fuzz/minimise_corpuses.sh`. Note that minimisation is, oddly, often not idempotent for unknown reasons.