Adding contribution note (#873)

Clarified contribution guidelines, and added extra notices.

Co-authored-by: Medeni Baykal <433724+Haplois@users.noreply.github.com>

.github/ISSUE_TEMPLATE.md

@@ -1,10 +1,13 @@
 ## Description
 > **IMPORTANT**: if the defect is reproduced only in a workflow from within the Visual Studio IDE then do not report the issue here - instead, please report it using Visual Studio's "Send Feedback" option that can be accessed from the Help menu OR using this link https://developercommunity.visualstudio.com.
 >
+> **IMPORTANT**: Before posting an issue, please search the repository for similar ones. It might have been solved or reported already. 
+> 
 > For a defect specific to the MSTest V2 test framework, describe the issue you've observed.
 
 ## Steps to reproduce
 > What steps can reproduce the defect?
+> 
 > Please share the setup, sample project, target platform (desktop, core, uap)
 
 ## Expected behavior
@@ -15,5 +18,7 @@
 
 ## Environment
 > Please share additional details about the test environment.
-> Operating system, Build version of vstest.console, Package version of MSTest
-> framework and adapter
+>  - Operating system
+>  - Build version of vstest.console
+>  - Package version of MSTest framework and adapter
+>  - Other installed packages and their versions on the test project