Update and revise contributing guide

[zekefarwell]

I updated the contributing guide to better match the state of the project. Removed out of date info, simplified some things, expanded on some other things. Hopefully it's an overall improvement.

[ZeLonewolf]

Looks good to me. I made some adjustments. Thanks Zeke for taking this on!

[adamfranco]

Sorry for being late to reviewing -- the updated CONTRIBUTING.md says to use npm install, but the dependencies are currently dev-dependencies, which require npm install --include=dev. We could make these normal dependencies if the intention is to not allow usage of Americana code by other projects(see #146), or we should update the docs to add the --include=dev. I could go either way.

[zekefarwell]

In my experience npm install installs both dependencies and devDependencies unless running in a production context. So adding --include=dev shouldn't be necessary when running the project for local development.

From the NPM Docs:

By default, npm install will install all modules listed as dependencies in package.json.

With the --production flag (or when the NODE_ENV environment variable is set to production), npm will not install modules listed in devDependencies. To install all modules listed in both dependencies and devDependencies when NODE_ENV environment variable is set to production, you can use --production=false.

More explanation here

When you (or another user) run npm install, npm will download dependencies and devDependencies that are listed in package.json

I think maybe in older versions of NPM it didn't work this way and you did have to explicitly include devDependencies. I haven't needed to since at least v6, though. Maybe even earlier.