[Proposal] Using ECE Tools with Magento Open Source

As ECE Tools became open source and public available - it's good idea to add instructions how to use it with Magento OS. As result - people might try to use it on Magento OS and will report issues / send PRs.
I just tried and got few issues:

- [ ] all docs are saying how to use it on Magento Commerce, but none are saying how to use them with Magento OS
- [x] package magento/ece-tools is not public available, even not available when I put my credentials w/o access to Magneto Commerce, there only one way to install it - use link to github repo as a composer repository

Start by reviewing the existing ECE Tools documentation for Magento Commerce and the installation guidance for magento/ece-tools. Add clear instructions for using ECE Tools with Magento Open Source, including the package-availability limitation described in the issue. Verify the documented process against a Magento Open Source installation.

Written by the indexing model from the issue text.