Add note when updating system.memory

[benvillalobos]

Context

We have customer projects break when we update system.memory, we should make it easier for customers to figure out these breakages when they're due to us updating system.memory.

Changes Made

Add a simple note on our PackageReference to System.Memory, with guidelines.

Testing

Notes

PR is WIP.

[benvillalobos]

Note to self: Also add the note to the relevant places here: https://github.com/dotnet/msbuild/pull/7680/files

[rainersigwald]

I think we should go forward with this path--maybe with even stronger warnings since it's subtle and painful. @BenVillalobos can you adopt some of the changes and we'll get it merged?

[benvillalobos]

@rainersigwald brought in your suggestion but couldn't find where we would file VS breaking changes. Feel free to merge since this is better than what we have today.

[benvillalobos]

Added link to internal guidelines, should be good to go