Mc knight 42/update contributing

[McKnight-42]

…pelines to Github Actions for CI testing.

resolves #https://github.com/dbt-labs/core-team/issues/29

Description

• Updated Contribute.MD to reflect swap over from CircleCI and Azure Pipelines to Github Actions.
• Made Changes to reflect Removal of Adapaters from dbt-core repo to own repos
• Added link and message to convey that Adapter specific issues/contributions should be made on their repos not the core repo.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Matthew McKnight.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[McKnight-42]

@cla-bot check

[kwigley]

Looks good to me 👍

[jtcohen6]

Nice work @McKnight-42! I left one suggestion. After addressing, feel free to merge without waiting for another review.