Add contributing guidelines

[LAShemilt]

Write some contributing guidelines to show which documentation goes where

[LAShemilt]

We can add a template / series of questions to guide people in making documentation in user-documentation. We could make this a GitHub Template

E.g.

• Describe your feature, what does it do
• how is this implemented in SciCat from a site-admin/operator perspective? E.g. which config files do you have to change. Add any examples you have.
• How does this look to the user?
• Troubleshooting/testing
• (nice to have) link to developer document