Add contributing guidelines

[chrisdmacrae]

Problems we're trying to solve

We want to publish a public version of this repository (only updated by CircleCI when changes are pushed to the private repository) that people can use to:

• Suggest changes to docs

• Suggest changes to blog posts

• Submit issues

• Submit showcase websites

• Add contributing guidelines

• Add license (closed-source, belongs to us)

• Add issue template

Issue template

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**- Do you want to request a *feature*, report a *bug*, or submit a website to the *showcase*?**

<!-- FEATURES ONLY: don't fill this out if it is not a feature -->

<!-- BUGS ONLY: don't fill this out if it is not a bug -->
**- What is the current behavior?**

**- If the current behavior is a bug, please provide the steps to reproduce.**

**- What is the behaviour you expect?**

<!-- SHOWCASE ONLY: don't fill this out if it is not a showcase submission -->
**- What is your website's URL?**

**- Is this a personal or commercial website?**

**- Are you the owner or maintainer of this website?**

**- What is your site's ID in Forestry?**
<!-- This can be found in the URL for the site: https://app.forestry.io/sites/{{ SITE_ID }}/

**- Provide the screenshot or `.gif` you'd like us to use (OPTIONAL)**