Dev docs: added "Unit tests"

Taken from https://github.com/koreader/koreader-base/wiki/Unit-testing with intent to delete

doc/Unit tests.md

@@ -0,0 +1,14 @@
+# Unit Tests
+
+Unit tests are automatically performed using [busted](http://olivinelabs.com/busted/). It depends on `luarocks`.
+
+To grab busted, install the same version [as used in the automated tests](https://github.com/koreader/koreader/blob/master/.ci/install.sh). At the time of writing that is 2.0.rc12-1:
+```bash
+mkdir $HOME/.luarocks
+cp /etc/luarocks/config.lua $HOME/.luarocks/config.lua
+echo "wrap_bin_scripts = false" >> $HOME/.luarocks/config.lua
+luarocks --local install busted 2.0.rc12-1
+```
+Then you can set up the environment variables with `./kodev activate`.
+
+If all went well, you'll now be able to run `./kodev test front` (for the frontend) or `./kodev test base` (for koreader-base).