Add installation guide

Closes #15

.vuepress/config.js

@@ -41,6 +41,7 @@ module.exports = {
 
     sidebarDepth: 3,
     sidebar: [
+      '/lib/Installation',
       '/lib/WhyUsePromises',
       '/lib/Tour',
       '/lib/Examples',

lib/Installation.md

@@ -0,0 +1,34 @@
+---
+title: Installation
+---
+
+# Installation
+
+### Method 1 - Quick and Dirty
+
+1. Make sure HTTP requests are enabled in Game Settings
+2. In Roblox Studio, select the folder where you keep your third party modules / utilities.
+3. Run this in the command bar:
+
+<textarea readonly style="width: 100%" onclick="this.select()">local m = Instance.new("ModuleScript") m.Parent = game:GetService("Selection"):Get()[1] or game:GetService("ServerScriptService") m.Name = "Promise" m.Source = game:GetService("HttpService"):GetAsync("https://raw.githubusercontent.com/evaera/roblox-lua-promise/master/lib/init.lua") game:GetService("Selection"):Set({m})</textarea>
+
+### Method 2 - Manual
+
+1. Visit the [latest release](https://github.com/evaera/roblox-lua-promise/releases/latest)
+2. Under *Assets*, click `Promise.lua`
+3. - Using [Rojo](https://rojo.space/)? Put the file into your game directly.
+   - Using Roblox Studio? Open the file, copy its contents, and paste into a ModuleScript and call it `Promise`.
+
+### Method 3 - Git Submodule
+
+1. Add the Promise repository as a git submodule (ideally within a folder called `submodules`) (tutorial [here](https://gist.github.com/gitaarik/8735255))
+2. Update your [Rojo configuration](https://rojo.space/docs/6.x/project-format/) to point to the appropriate path and sync the file in.
+
+### Method 4 - Package Manager
+
+Using a package manager? The repository has a [`rotriever.toml`](https://github.com/evaera/roblox-lua-promise/blob/master/rotriever.toml).
+
+
+## Next
+
+Now, check out the [API reference](/lib)!