Documentation on kernel addition

[dkisselev]

Most SF bundles add a bit to their install guide about registering the bundle in the app kernel.

Although it's not mandatory, as a user it really helps me because it saves me having to remember to do that and needing to go look up your exact bundle namespace.

Took me a few minutes of "why does SF keep telling me the omnipay service doesn't exist" to figure it out.

Here's how it looks for KnpPaginator: https://github.com/KnpLabs/KnpPaginatorBundle#add-paginatorbundle-to-your-application-kernel