Improving README a little bit

[Antonio-Laguna]

Fixes #59

[vojtajina]

We discussed this a couple of times. The recommended way is to install karma and all the plugins your project needs locally in that project (and putting them into package.json). That way, other developers can just do npm install and everything is set up.

If you follow that strategy, you should not be required to specify "plugins" in karma.conf.js - Karma will load all these plugins by default.

What is the use case where this does not work?

[aymericbeaumet]

@Belelros It's been 2 months, reopen this PR if you still think it could be valuable.