Add CLI basics in docs

Summary:
Adding documentation for the CLI for 2 reasons:

1.  It's not immediately clear to most how the CLI is part of `react-native` and not the node module they installed.  This begins clarifying.
2. I plan on adding some additional documentation to this section pending an upcoming PR I've discussed with Mike.

screen shot provided:
![image](https://cloud.githubusercontent.com/assets/997157/20574213/0640d026-b179-11e6-9bc1-4f507b2a195a.png)
Closes #11097

Differential Revision: D4237028

Pulled By: mkonicek

fbshipit-source-id: c1dc50fe1be7f6947a33ee6472b5862306888055

docs/Performance.md

@@ -4,7 +4,7 @@ title: Performance
 layout: docs
 category: Guides
 permalink: docs/performance.html
-next: upgrading
+next: understanding-cli
 previous: navigation
 ---
 

docs/UnderstandingCLI.md

@@ -0,0 +1,38 @@
+---
+id: understanding-cli
+title: Understanding the CLI
+layout: docs
+category: Guides
+permalink: docs/understanding-cli.html
+next: upgrading
+previous: performance
+---
+
+Though you may have installed the `react-native-cli` via npm as a separate module, it is a shell for accessing the CLI embedded
+in the React Native of each project. Your commands and their effects are dependent on the version of the module of `react-native`
+in context of the project. This guide will give a brief overview of the CLI in the module.
+
+# The local CLI
+
+React Native has a [`local-cli`](https://github.com/facebook/react-native/tree/master/local-cli) folder with a file named
+[`cliEntry.js`](https://github.com/facebook/react-native/blob/master/local-cli/cliEntry.js).  Here, the commands are read
+from `commands.js` and added as possible CLI commands.  _E.G._ the `react-native link` command, exists in the
+[`react-native/local-cli/link`](https://github.com/facebook/react-native/blob/master/local-cli/link/) folder, and is
+required in `commands.js`, which will register it as a documented command to be exposed to the CLI.
+
+# Command definitions
+
+At the end of each command entry is an export.  The export is an object with a function to perform, description of the command, and the command name.  The object structure for the `unlink` command looks like so:
+
+```js
+module.exports = {
+  func: link,
+  description: 'links all native dependencies',
+  name: 'link [packageName]',
+};
+```
+
+### Parameters
+
+The command name identifies the parameters that a command would expect.  When the command parameter is surrounded by greater-than, less-than symbols `< >`, this indicates that the parameter is expected.  When a parameter is surrounded by brackets `[ ]`, this indicates that the parameter is optional.
+

docs/Upgrading.md

@@ -5,7 +5,7 @@ layout: docs
 category: Guides
 permalink: docs/upgrading.html
 next: platform-specific-code
-previous: performance
+previous: understanding-cli
 ---
 
 Upgrading to new versions of React Native will give you access to more APIs, views, developer tools