tools/emul: add troubleshooting instructions in README

fixes #79

tools/emul/README.md

@@ -57,3 +57,18 @@ memory starting at address 0 and then run the code until it halts. The exit
 code of the program is the value of `A` when the program halts.
 
 This is used for unit tests.
+
+## Problems?
+
+If the libz80-wrapped zasm executable works badly (hangs, spew garbage, etc.),
+it's probably because you've broken your bootstrap binaries. They're easy to
+mistakenly break. To verify if you've done that, look at your git status. If
+`kernel.bin` or `zasm.bin` are modified, try resetting them and then run
+`make clean all`. Things should go better afterwards.
+
+If that doesn't work, there's also the nuclear option of `git reset --hard`
+and `git clean -fxd`.
+
+If that still doesn't work, it might be because the current commit you're on
+is broken, but that is rather rare: the repo on Github is plugged on Travis
+and it checks that everything is smooth.