Contributing Doc (mmapv1 deprecated in mongodb 4.2)

[FlameFractal]

Do you want to request a feature or report a bug?

Documentation fix

What is the current behavior?

CONTRIBUTING.md states

- start a mongodb instance on port 27017 if one isn't running already. `mongod --dbpath <path to store data> --port 27017 --storageEngine mmapv1`. Mongoose tests run much faster on the mmapv1 storage engine as opposed to the WiredTiger storage engine.

As mmapv1 has been removed in MongoDB 4.2, starting the server with the command in the doc gives error Cannot start server with an unknown storage engine: mmapv1

We should probably update the doc to reflect the same.

[vkarpov15]

Will do. We did a lot of work to make Mongoose tests not create too many collections: rapidly creating and dropping collections makes WiredTiger grind to a halt. Right now we typically run tests locally using the inMemory storage engine on MongoDB enterprise.