Update README

[jankapunkt]

Add the description about testing in the README:

You can run the tests like the following:

$ cd js-set-extension/package
$ npm install

To run tests in watch mode use

$ npm run test-watch

or for a single run use

$ npm run test

The tests are written in mocha but it should not be that hard to get into it as it is written very intuitive.

Linting

Please note, that the tests are very strict about code style and you can check for code style related errors using

npm run lint

You should fix these lint errors, since the CI server will reject to run any tests when the linter as thrown an error.

You can also run lint and tests all in one process using

npm run lint-test

The scripts to run these commands are also in the package.json file:

https://github.com/jankapunkt/js-set-extension/blob/master/package/package.json

How to update the code to this PR

If you make updates to your forked repo on the branch you have opened this PR (it is MareoRaft:master as you can see in the title) it will automatically be added to this PR and the CI server will also run the tests again. So no worries in changing and updating your code here.