Add extends queryBuilder documentation - related to #3334

[felixmosh]

No description provided.

[kibertoad]

@felixmosh Can you also add documentation for extending the TS QueryBuilder interface?

[kibertoad]

Review comments added.

[cjnqt]

If I understand correctly, the docs show the .extend function being used before initialisation of the connection.

But how do you extend the query builder and then initialise the connection? knex.initialize(config) doesn't work (seems to be deprecated)

[kibertoad]

@cjnqt From my understanding you don't need to do it in some very spefic moment, as extension is quite global. You can do it once during your application initialization, before you start building any queries, and then you are good.

[elhigu]

knex.initialize(config) should work at least I haven't heard it being deprecated. Though it is only needed if you call knex.destroy() and after that you need to reinitialize the same knex instance with new pool.