Updates to join-mainnet.md

[faddat]

Description

Adds an instant-gratification code snipped to join-mainnet.md

We may wish to briefly discuss the file's other contents before merging, I removed obviously outdated information so far.

---

For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

---

For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[shahankhatch]

Merged before seeing your phrase 'We may wish to briefly discuss the file's other contents before merging'. Apologies!

[faddat]

@shahankhatch it's okay, I think it'll work out fine :).