Create a reporting_guidelines.md

## Description
As a user, I need to modify the reporting guidelines, so that I can seek for help when necessary.
1. The content should be consistent with [Reporting Guidelines](https://github.com/anitab-org/mentorship-flutter/blob/develop/.github/reporting_guidelines.md)
2. And then update the code of conduct link in **Readme.md**>>Contributing section.
3. Add the updated Reporting Guidelines file to doc folder.

## Acceptance Criteria
- [ ] Update [Required]

## Definition of Done
- [ ] All of the required items are completed.
- [ ] Approval by 1 mentor.

## Estimation
30 minutes

## Note
This issue will be assigned from September 1, 2020.