Tip contributors about 500 from x-api after build

README.md

@@ -158,6 +158,34 @@ cd ~/workspace/exercism/xjava
 ./bin/build.sh
 ```
 
+## Developing
+
+### Tip: gradle clean before exercism fetch
+
+If you `exercism fetch` after doing a build, the CLI will fail with the following error message:
+
+```
+$ exercism fetch java bob
+2015/09/06 15:03:21 an internal server error was received.
+Please file a bug report with the contents of 'exercism debug' at: https://github.com/exercism/exercism.io/issues 
+```
+
+and if you review the logs of your x-api, you'll find:
+
+```
+127.0.0.1 - - [06/Sep/2015:15:20:56 -0700] "GET /v2/exercises/java/bob HTTP/1.1" 500 514949 0.2138
+2015-09-06 15:21:01 - JSON::GeneratorError - source sequence is illegal/malformed utf-8:
+```
+
+This is because some files generated by the build can't be served from the x-api.  That's actually fine.  To fix this, simply make sure you do a clean in your xjava repo before you fetch:
+
+```
+cd ~/workspace/exercism/xjava
+gradle clean
+cd ~/worksapce/exercism/exercises
+exercism fetch java bob
+```
+
 ## Submitting Your Contribution
 
 First of all... Exercism is meant help programmers, world-wide, to develop their skills and in doing so help raise the bar on our industry as a whole.  Your contribution is making that experience even better.  Thank you!