Update the README with basic instruction for getting started

[ci skip]

CONTRIBUTING.md

@@ -1,4 +1,4 @@
 Contributing
-======
+============
 
 Information on contributing to this repo is in the [Contributing Guide](https://github.com/aspnet/Home/blob/dev/CONTRIBUTING.md) in the Home repo.

README.md

@@ -1,12 +1,17 @@
 KestrelHttpServer
 =================
 
-[![Join the chat at https://gitter.im/aspnet/KestrelHttpServer](https://badges.gitter.im/aspnet/KestrelHttpServer.svg)](https://gitter.im/aspnet/KestrelHttpServer?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
-
 AppVeyor: [![AppVeyor](https://ci.appveyor.com/api/projects/status/nr0s92ykm57q0bjv/branch/dev?svg=true)](https://ci.appveyor.com/project/aspnetci/KestrelHttpServer/branch/dev)
 
 Travis: [![Travis](https://travis-ci.org/aspnet/KestrelHttpServer.svg?branch=dev)](https://travis-ci.org/aspnet/KestrelHttpServer)
 
 This repo contains a cross-platform web server for ASP.NET Core.
 
 This project is part of ASP.NET Core. You can find samples, documentation and getting started instructions for ASP.NET Core at the [Home](https://github.com/aspnet/home) repo.
+
+## Building from source
+
+Before opening this project in Visual Studio or VS Code, execute `build.cmd /t:Restore` (Windows) or `./build.sh /t:Restore` (Linux/macOS).
+This will download and initialize a few required build tools and packages.
+
+See [developer documentation](https://github.com/aspnet/Home/wiki) for more details.