Simplify README.md, add extra postfix to developer build images.

Code-Reviewer Section

The general guidelines can be found here.

Please check each of the following things and check all boxes before accepting a PR.

• The PR has a description, explaining both the problem and the solution.
• The description mentions which forms of testing were done and the testing seems reasonable.
• Every function/class/actor that was touched is reasonably well documented.

For Release-Branches

If this PR is made against a release-branch, please also check the following:

• This change/bugfix is a cherry-pick from the next younger branch (younger release-branch or master if this is the youngest branch)
• There is a good reason why this PR needs to go into a release branch and this reason is documented (either in the description above or in a linked GitHub issue)