Specify file name where custom application root is required when using Spring with a Rails engine

[boddhisattva]

Background:

When attempting to using Spring with a Rails engine, I wasn't quite sure in which file do I need to include Spring.application_root = './test/dummy' in order to use it. I found the answer after some googling around via this link. IMHO it could be useful to have the same info in the README so that people exactly know how to use Spring in the context of a special setup like a Rails engine.

[rails-bot]

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