Add note about building TS library first (#2324)

Author: analogrelay

clients/ts/DebuggingTests.md

@@ -6,6 +6,14 @@ Prerequisites: NodeJS, have run `./build.cmd /t:Restore` at least once since cle
 
 All commands must be run from this directory (the `clients/ts` directory).
 
+## Building the library before running tests
+
+You need to build the libraries any time you make a change, before you can run the tests. Do this by running the following command from the `clients/ts` folder:
+
+```
+> npm run build
+```
+
 ## Running all tests
 
 ```
@@ -91,4 +99,4 @@ You can launch all tests under the debugger in Visual Studio Code by clicking on
 
 You can launch all tests **in the currently open file** under the debugger in Visual Studio Code by clicking on the "Debug" tab on the left side, selecting "Jest - Current File" in the dropdown at the top and clicking the play button, or pressing F5.
 
-**NOTE**: Pair this with `.only` to easily debug a single test!
+**NOTE**: Pair this with `.only` to easily debug a single test!