add code signing tutorial (#12767)

* docs: add code signing tutorial

* docs: link to code signing tutorial from readme

* docs: remove unneeded use of "application" from docs readme

docs/tutorial/code-signing.md

@@ -0,0 +1,46 @@
+# Code Signing
+
+Code signing is a security technology that you use to certify that an app was 
+created by you. Once an app is signed, the system can detect any change to the 
+app, whether the change is introduced accidentally or by malicious code.
+
+While it is possible to distribute unsigned apps, it is not recommended. 
+For example, here's what macOS users see when attempting to start an unsigned app:
+
+![unsigned app warning on macOS](https://user-images.githubusercontent.com/2289/39488937-bdc854ba-4d38-11e8-88f8-7b3c125baefc.png)
+
+> App can't be opened because it is from an unidentified developer
+
+If you are building an Electron app that you intend to package and distribute, 
+it should be code signed. The Mac and Windows app stores do not allow unsigned 
+apps.
+
+# Signing macOS builds
+
+Before signing macOS builds, you must do the following:
+
+1. Enroll in the [Apple Developer Program](Apple Developer Program) (requires an annual fee)
+2. Download and install Xcode
+3. Generate, download, and install [signing certificates]
+
+There are a number of tools for signing your packaged app:
+
+- [`electron-osx-sign`] is a standalone tool for signing macOS packages.
+- [`electron-packager`] bundles `electron-osx-sign`. If you're using `electron-packager`,
+pass the `--osx-sign=true` flag to sign your build.
+- [`electron-builder`] has built-in code-signing capabilities. See [electron.build/code-signing](https://www.electron.build/code-signing)
+
+For more info, see the [Mac App Store Submission Guide].
+
+# Signing Windows builds
+
+See the [Windows Store Guide].
+
+[Apple Developer Program]: https://developer.apple.com/programs/
+[`electron-osx-sign`]: https://github.com/electron-userland/electron-osx-sign
+[`electron-packager`]: https://github.com/electron-userland/electron-packager
+[`electron-builder`]: https://github.com/electron/electron-builder
+[Xcode]: https://developer.apple.com/xcode
+[signing certificates]: https://github.com/electron-userland/electron-osx-sign/wiki/1.-Getting-Started#certificates
+[Mac App Store Submission Guide]: mac-app-store-submission-guide.md
+[Windows Store Guide]: windows-store-guide.md