docs: add Pull Request Template

[petermetz]

Description

As a contributor I want to know in advance, before opening a pull request that commit linting is in effect so that I don't get my PR marked as failing just because I didn't know that the commit message was supposed to be in a certain format.

Create a Pull Request template as explained here: https://help.github.com/en/github/building-a-strong-community/creating-a-pull-request-template-for-your-repository

Acceptance Criteria

1. Pull Request template explains that commit linting is in effect and provides a link to the original specs => https://www.conventionalcommits.org/en/v1.0.0-beta.4/#specification
2. PR template has a few words about the git sign off requirement as well (and a link to guide) just in case someone is new to it and potentially gets all confused about why their PR is marked red.
3. The PR template also must explain that the pull requests should be rebased onto master and squashed into a single commit to help maintainers review it more efficiently and to avoid spaghetti git commit graphs that obfuscate which commit did exactly what change, when and why (which is literally the most important feature of a source code version control system such as git).
4. PR template includes a link to this very well written guide as a "must read" item for beginners: https://github.com/servo/servo/wiki/Beginner's-guide-to-rebasing-and-squashing

[petermetz]

FYI: There's a discussion on the Hyperledger Technical Steering Committee's mailing list about contributor on-boarding at the moment, might want to read through that thread and the final outcome of it later as well while working on this task.

[ruzell22]

I would like to work on this one, thank you