Update CONTRIBUTING.md

Author: vors

CONTRIBUTING.md

@@ -5,3 +5,22 @@ There are two permanent brances: **master** and **dev**.
 **Master** always points to the latest release tag. 
 All development is done in **dev** branch, 
 please, use it for pull requests.
+
+
+Run tests locally
+-----------------
+The most up-to date version can be crafted by following [appveyor tests steps](appveyor.yml).
+Currently tests use hardcoded path for a sublime text installation `C:\st`. 
+You can use the same script [appveyour.ps1](https://github.com/randy3k/UnitTesting/blob/master/sbin/appveyor.ps1) for installing sublime and tests execution.
+Separate installation used to avoid accident breaking the main installtion.
+
+One time install:
+
+1. download [appveyour.ps1](https://github.com/randy3k/UnitTesting/blob/master/sbin/appveyor.ps1).
+2. run one-time bootstrap: `.\appveyor.ps1 "bootstrap" -verbose` to install ST in `C:\st` and download [UnitTesting](https://github.com/randy3k/UnitTesting) package.
+3. Install PowerShell unit testing framework [Pester](https://github.com/pester/Pester).
+
+Run tests:
+
+1. Run python tests `.\appveyor.ps1 "run_tests" -verbose`. They generate inputs for pester tests.
+2. Run pester tests `Invoke-Pester`.