Document "How to work on wrangler" for CONTRIBUTING.md

[xortive]

We should provide a quick intro to some dev practices for wrangler. Things like building wrangler locally and installing/running your local build may not be intuitive for those not familiar with rust already.

We should provide a high-level map of the code that shows where basic high-level functionality is implemented. main.rs may serve this function adequately, but we can review if a separate map is helpful.

• Include info on --debug flag for building Document --debug flag for local builds #264.