I didn't think it was possible but it has been for years; to share a file between projects in a solution.
Add a file the normal way but instead of pressing Open, use the little dropdown arrow next to. Then you have the possibility to "link" instead of "add".
This makes it possible to have one and only one file for many places. It can be a source code file or a readme or anything really.
I have yet to try it with TFS and SVN to make sure it doesn't mess upp the version management.
On a side note: Sourcesafe has linking ability within itself. I miss it in TFS. Though: It is easy to lose the master file and keep copies wich makes strange icons. It also doesn't play well with the VB6 editor when a file will be checked out and another not, but they should be, but the VB6 ide doesn't understand it and you have to do an explicit checkout and... never mind. It was a long time ago.