Document package validation handling (#9216)

* Document package validation handling

* Move to contributing.md

* Add title

Author: sebastienros

docs/contributing.md

@@ -75,3 +75,17 @@ Make sure you have started Docker before trying to run an Aspire app.
 For information on who can help in PRs and issues, see the [area owners](area-owners.md) page.
 
 See the [tips and known issues](tips-and-known-issues.md) page.
+
+### Package validation
+
+When creating a new integration, package validation will automatically try to download a previous version of the package to ensure you didn't break compat. As a result you might get the following build error:
+
+```shell
+error NU1101: Unable to find package [NEW PACKAGE NAME]. No packages exist with this id in source(s): dotnet-eng, dotnet-public, dotnet9, dotnet9-transport. PackageSourceMapping is enabled, the following source(s) were not considered: dotnet-libraries.
+```
+
+To prevent this the new package needs this line to be added to the `.csproj`:
+
+```xml
+<EnablePackageValidation>false</EnablePackageValidation>
+```