Update README.md

[adnanhashmi09]

This pull request includes changes to the README.md file to update the instructions for storing credentials. The most important changes are:

Updates to credential storage instructions:

• Changed the instructions to add MHQ_CLIENT_ID and MHQ_CLIENT_SECRET to .env instead of client.txt.
• Changed the instructions to add GITLAB_DOMAIN_URL and GITLAB_ACCESS_TOKEN to .env instead of client.txt.
• Changed the instructions to add BITBUCKET_ACCESS_TOKEN and BITBUCKET_WORKSPACE_NAME to .env instead of client.txt.