Add docs for GraphQL Federation

[rickdgeerling]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[X] Other... Please describe: Documentation

What is the current behavior?

Issue Number: N/A

What is the new behavior?

N/A

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

Documents the Federation module from nestjs/graphql#455

[kamilmysliwiec]

cc @johnbiundo can you review when you have some time? it just got unlocked :)

[johnbiundo]

I made a number of small suggestions (style and grammar).

Also, I believe the .html file should not be in the commit (@kamilmysliwiec ?)

[rickdgeerling]

Suggestions accepted :) thank you!

You're right about not including the .html file. It's ignored by default so I have no idea how it ended up in there.

[gperdomor]

This need more work or can be merged?

[gperdomor]

ping @kamilmysliwiec

[jarvisuser90]

I found some typos.

[rickdgeerling]

@jarvisuser90 Thank you!

[kamilmysliwiec]

Thanks!