Add CONTRIBUTING.md. (#1456)

Author: sean-mcmanus

CONTRIBUTING.md

@@ -0,0 +1,18 @@
+# How to Contribute Changes
+
+## Contribution Steps:
+  * [Build and debug the extension](Documentation/Getting%20started.md#build-and-debug-the-cpptools-extension).
+  * File an [issue](https://github.com/Microsoft/vscode-cpptools/issues) and a [pull request](https://github.com/Microsoft/vscode-cpptools/pulls) with the change and we will review it.
+  * If the change affects functionality, add a line describing the change to [CHANGELOG.md](Extension/CHANGELOG.md).
+  * Try and add a test in [test/extension.test.ts](Extension/test/unitTests/extension.test.ts).
+  * Run tests via opening the [Extension](https://github.com/Microsoft/vscode-cpptools/tree/master/Extension) folder in Visual Studio Code, selecting the `Launch Tests` configuration in the Debug pane, and choosing `Start Debugging`.
+
+## About the Code
+  * Execution starts in the `activate` method in [main.ts](Extension/src/main.ts).
+    * `processRuntimeDependencies` handles the downloading and installation of the OS-dependent files. Downloading code exists in [packageManager.ts](Extension/src/packageManager.ts).
+    * `downloadCpptoolsJsonPkg` handles the `cpptools.json`, which can be used to enable changes to occur mid-update, such as turning the `intelliSenseEngine` to `"Default"` for a certain percentage of users.
+  * The debugger code is in the [Debugger](https://github.com/Microsoft/vscode-cpptools/tree/master/Extension/src/Debugger) folder.
+  * [LanguageServer/client.ts](Extension/src/LanguageServer/client.ts) handles various language server functionality.
+  * [LanguageServer/configurations.ts](Extension/src/LanguageServer/configurations.ts) handles functionality related to `c_cpp_properties.json`.
+  * [telemetry.ts](Extension/src/telemetry.ts): Telemetry data gets sent to either `logLanguageServerEvent` or `logDebuggerEvent`.
+  * The Tag Parser (symbol database) doesn't automatically expand macros, so the [cpp.hint](Extension/cpp.hint) file contains definitions of macros that should be expanded in order for symbols to be parsed correctly.