consensus: comment about test_data [ci skip]

consensus/replay_test.go

@@ -37,6 +37,12 @@ func init() {
 // the `Handshake Tests` are for failures in applying the block.
 // With the help of the WAL, we can recover from it all!
 
+// NOTE: Files in this dir are generated by running the `build.sh` therein.
+// It's a simple way to generate wals for a single block, or multiple blocks, with random transactions,
+// and different part sizes. The output is not deterministic, and the stepChanges may need to be adjusted
+// after running it (eg. sometimes small_block2 will have 5 block parts, sometimes 6).
+// It should only have to be re-run if there is some breaking change to the consensus data structures (eg. blocks, votes)
+// or to the behaviour of the app (eg. computes app hash differently)
 var data_dir = path.Join(cmn.GoPath, "src/github.com/tendermint/tendermint/consensus", "test_data")
 
 //------------------------------------------------------------------------------------------