chore: add doc about adding new states to context

[kasir-barati]

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
• Docs
• Other... Please describe:

What is the current behavior?

Just add a bit of doc about how you can attach request/response, or other stuff to your context object.

Issue Number: #2143

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kasir-barati]

Ready for code review.

Some off-topic comments:

1. Can we switch to pnpm? Is that doable for this repo? (saves space, net, and more importantly time).
2. Can we update README.md of this repo to something that tells people how to run and test it on their local machine? Right now it is just your normal README.md that you'll get when creating a new NestJS app with Nest CLI.

[kasir-barati]

JFYI, this question and the linked issue led to this PR 🙂.