(doc) Add testing section to link to other docs

It was pointed out in community discussion that the main README file
doesn't mention anything to do with running tests for the Chocolatey
CLI codebase.  This commit adds a new Testing section to the README.md
file, that links off to the pertinent information in the TESTING.md
file.

Used the Markdown All in One VSCode extension to update the table of
contents.  Initially this was adding an entry in the TOC section for the
first heading, which was not there before, so added an entry in
workspace settings.json file to exclude this.  Updating this table of
contents actually picked up on some previously entries which hadn't
been added.

.vscode/settings.json

@@ -1,5 +1,6 @@
 {
     "[powershell]": {
         "files.encoding": "utf8bom",
-    }
+    },
+    "markdown.extension.toc.levels": "2..6"
 }

README.md

@@ -27,7 +27,10 @@ You can just call me choco.
     - [Windows](#windows)
     - [Other Platforms](#other-platforms)
       - [Prerequisites:](#prerequisites)
+      - [Before building:](#before-building)
       - [Build Process:](#build-process)
+  - [Testing](#testing)
+  - [Installing on Other Platforms:](#installing-on-other-platforms)
 - [Credits](#credits)
 
 <!-- /TOC -->
@@ -200,6 +203,15 @@ chmod +x *.sh
 
 Running the build on Mono produces an artifact similar to Windows but may have more rough edges. You may get a failure or two in the build script that can be safely ignored.
 
+### Testing
+
+The Chocolatey CLI codebase contains a number of different tests, including unit, integration, and end-to-end tests.
+These are all documented in the [TESTING.md](https://github.com/chocolatey/choco/blob/develop/TESTING.md) file.
+
+The unit and integration tests are NUnit tests, which can be executed directly within Visual Studio, or as part of the [build process](https://github.com/chocolatey/choco/blob/develop/TESTING.md#running-tests).
+
+The end-to-end tests use Pester as the test framework. Since these tests have the potential to actually make changes to your system, we recommend using the [provided Vagrant file to run these tests in a dedicated virtual machine](https://github.com/chocolatey/choco/blob/develop/TESTING.md#pester-tests).
+
 ### Installing on Other Platforms:
 
  1. Get a copy of the source code and build.