Improve "Post merge" instructions in Readme

[carlossanlop]

@ViktorHofer explained the process after merging a new package. We need to add those instructions in the readme "post merge" section: https://github.com/dotnet/runtime-assets#2-post-merge

Summarized:

• After merge, wait for the pipeline to finish and the packages get published to Darc.
• Look at the runtime-assets-official artifacts list, the new package should be there.
• Copy the version of the package.
• In the runtime repo, add entries for the new package to Versions.props and to Versions.Details.xml

[ViktorHofer]

Sounds good. But I would not share any links to internal builds or the public dotnet7-transport feed as that is subject to change. I would just call them out but not link to them as currently done in the instructions.