isogram: add hint for german umlauts encoding

exercises/isogram/HINT.md

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+If you find that `testWorksWithGermanLetters` fails even though you think you've implemented it correctly,
+this could be because of the [JVM encoding](https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_61/rzaha/international.htm). 
+Try setting the `JAVA_TOOL_OPTIONS` environment variable to `-Dfile.encoding=UTF8` to fix this.
+- To set an environment variable on Windows:
+    1. Go to Computer -> Properties -> Advanced System Settings
+    2. Add a new environment variable
+
+- [To set an environment variable on a mac](http://blog.lidalia.org.uk/2011/04/setting-default-java-file-encoding-to.html)
+
+- [To set an environment variable on linux](https://unix.stackexchange.com/questions/151733/where-can-i-set-global-java-options)