Add installation instructions

[glensc]

Not sure how bats extensions are supposed to be installed in bats eco-system.

bats-core has multiple mechanisms documented:

• https://github.com/bats-core/bats-core#installation

but nothing in their readme how to install other adds-ons:

• https://github.com/bats-core/bats-core#libraries-and-add-ons

so, as both projects have package.json:

• https://github.com/bats-core/bats-core/blob/v1.2.0/package.json
• https://github.com/bats-core/bats-file/blob/v0.3.0/package.json

perhaps this community is npm based, but then the bats-file is not present on npm:

[/tmp] ➔ yarn add bats-file
yarn add v1.22.4
info No lockfile found.
[1/4] 🔍  Resolving packages...
error An unexpected error occurred: "https://registry.yarnpkg.com/bats-file: Not found".
info If you think this is a bug, please open a bug report with the information provided in "/private/tmp/yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.

[simar7]

You could try using brew if you're on a Mac to install. brew install bats-file. I agree, it should be documented somewhere.