feat(test): Tests for custom install dir

ref: issue #34

README.md

@@ -26,12 +26,22 @@ _Note:_
 
  - Only compatible with fisher v4 upwards; v3 is no longer supported.
  - You have to install [SDKMAN!] separately.
+ - If you have installed SDKMAN! at a custom location, you need to add
+   ```fish
+   set -g __sdkman_custom_dir /your/path/to/sdkman
+   ```
+   to a fish config file
+     [run _before_](https://fishshell.com/docs/current/language.html#configuration-files)
+   `.config/fish/conf.d/sdk.fish`;
+   for example, you can use `.config/fish/conf.d/config_sdk.fish`.
+
 
 ## Usage
 
 It's all in the background; you should be able to run `sdk` and binaries installed
 with `sdk` as you would expect.
 
+
 ## Contribute
 
 When you make changes, 
@@ -64,6 +74,7 @@ done
 wait
 ```
 
+
 ## Acknowledgements
 
  * Completion originally by [Ted Wise](https://github.com/ctwise); see his