Replace broken link for commit guidelines

[KNWR]

Motivation

The contribution guidelines suggest that contributors Ensure your [commit messages are well-written](https://medium.com/brigade-engineering/the-secrets-to-great-commit-messages-106fc0a92a25).

The medium link redirects to an expired domain.

Solution

This PR replaces the broken link with the latest version saved by archive.org: https://web.archive.org/web/20160316232453/https://medium.com/brigade-engineering/the-secrets-to-great-commit-messages-106fc0a92a25

I've opened a PR as this is a quick fix, but happy to open an issue instead if that's preferred.

In other repos

The rosetta website and other rosetta repos also use the broken link (in rosetta-cli, in rosetta-sdk-go, in rosetta-bitcoin). If this PR is helpful, I'll make the same fix in the other rosetta repos, unless there's a better solution.

[heimdall-asguard]

If Heimdall is stuck, please click the following link: https://heimdall-public.cbhq.net/v1/pull_request/stuck?pr_number=58&repository=coinbase/rosetta-specifications

[patrick-ogrady]

Great catch @KNWR ! Let's actually switch to using this page instead: https://chris.beams.io/posts/git-commit/#seven-rules

Updating the other repos with this would be AWESOME!

[patrick-ogrady]

@KNWR Looks like you'll need to sign your commit before I can review 😢

[patrick-ogrady]

Had to close and open PR to kickoff build for some reason...

[patrick-ogrady]

This is an annoying one 😠 ... @KNWR looks like you'll need to remove the unsigned commit to merge:

All other PRs are approved and merged 🚀

[KNWR]

My mistake, I meant to amend the unsigned commit, not create another one. Just fixed it.