Add information about usage in plugins

[julkue]

As this topic describes http://stackoverflow.com/questions/25847986/no-provider-for-jasmine-jquery it is not possible to use karma-jasmine-jquery without adding it into the "plugins" array. This is not described in the documentation.

[bessdsv]

i think people who use a karma and know about plugins must know how enable plugin(framework). It is in karmas' manual. but i can add it to example of using.

[julkue]

Well for me for example this is not trivial and was not clear at first.

[clintonb]

Adding this information to the README would have saved me 10 minutes of frustration. I don't deal with front-end work very often, so I rely on READMEs. As it is, your README only provides half of the information needed to get started. The extra 5 lines with a plugins definition will save a lot of hassle.