[Documentation] update the CONTRIBUTING.md with Issues & Pull Requests sections

[sandramsc]

Current Behavior

The contributing guide is a great way to introduce new and existing contributors on how to add their contributions to the projects within an organization. Currently, the CONTRIBUTING.md does not state that:

• there is a first-come-first-serve procedure when it comes to assigning open issues.
• allocated time to update the community on your progress

Desired Situation

Add/update the following sections within the CONTRIBUTING.md of the BlackPythonDevs projects:

• Issues & Pull Requests --> i.e Creating an Issue, Working on an issue (get it assigned to you)
• Reviewing Pull Requests

Checklist for projects

All the CONTRIBUTING.md's need to be updated using above mentioned desired state.

• BlackPythonDevs
• Streamlit Booth Slider

[kjaymiller]

I'm looking forward to this contribution!