Update README

[danilo04]

Fix

Update README file with the following changes:

• Modify gradle command to install debug version of Simplenote
• Add Setup Credentials section
• Add Contribution guidelines section
• Add section to mention how to run tests

Review

Only one developer and one designer are required to review these changes, but anyone can perform the review.

Release

These changes do not require release notes.

[peril-automattic[bot]]

You can test the changes on this Pull Request by downloading the APK here.