clarify use of path for sidedoor.get

[AndrewSouthpaw]

If using a path to locate the module, sidedoor.get currently requires the module pathname to be written relative from the location of sidedoor.js. For example, if sidedoor is installed via npm, then a module at the root project folder would need to be referenced this way: sidedoor.get('../../../my-module'). This is because require.resolve is called from within that file.

This issue can be avoided by simply passing in an absolute path. The distinction is clarified in the README and an example provided on how to pass an absolute path.