Move 'advanced gradients' wiki in readme

Author: bokub

README.md

@@ -45,8 +45,6 @@ let coolGradient = gradient([
 ]);
 ```
 
-You can check out more advanced usages of gradients in the [wiki](https://github.com/bokub/gradient-string/wiki/Advanced-gradients), such as custom color stops or color interpolation options. 
-
 ### Use a gradient
 
 ```javascript
@@ -57,15 +55,86 @@ console.log(coolString);
 ## Built-in gradients
 
 ### Usage
+
 ```javascript
 const gradient = require('gradient-string');
 
 // Use the rainbow gradient
 gradient.rainbow('I love gradient-strings!')
 ```
-#### Available built-in gradients
+
+### Available built-in gradients
 [![Built-in gradients](http://bit.ly/2uFygrL)](http://bit.ly/2ufX07r)
 
+## Advanced gradients
+
+<details>
+  <summary>
+    There are also more advanced options for gradient customization, such as custom color stops, or choice of color interpolation
+  </summary>
+  
+### Custom color stops
+
+By default, the gradient color stops are distributed equidistantly.
+
+You can specify the position of each color stop (between `0` and `1`), using the following syntax:
+
+```javascript
+let coolGradient = gradient([
+  {color: '#d8e0de', pos: 0},
+  {color: '#255B53', pos: 0.8},
+  {color: '#000000', pos: 1}
+]);
+```
+
+### Color interpolation
+
+When using a gradient, you can actually add a second parameter to choose how the colors will be generated.
+
+Here is the full gradient API:
+
+#### myGradient(text, [options])
+
+##### text
+Type: `string`<br>
+String you want to color.
+
+##### options
+Type: `Object`<br>
+
+###### interpolation
+Type: `string`<br>
+The gradient can be generated using RGB or HSV interpolation. HSV usually produces brighter colors.
+`interpolation` can be set to `rgb` for RGB interpolation, or`hsv` for HSV interpolation.<br>
+Defaults to `rgb`. Case insentitive
+
+###### hsvSpin
+Type: `string`<br>
+Used only in the case of HSV interpolation.<br>
+Because hue can be considered as a circle, there are two ways to go from a color to another color.<br>
+`hsvSpin` can be either `short` or `long`, depending on if you want to take the shortest or the longest way between two colors.<br>
+Defaults to `short`. Case insensitive
+
+#### Example
+##### Code
+```javascript
+const redToGreen = gradient('red', 'green');
+const str = '■'.repeat(48);
+
+// Standard RGB gradient
+console.log(redToGreen(str)); 
+
+// Short HSV gradient: red -> yellow -> green
+console.log(redToGreen(str, {interpolation: 'hsv'}));
+
+// Long HSV gradient: red -> magenta -> blue -> cyan -> green
+console.log(redToGreen(str, {interpolation: 'hsv', hsvSpin: 'long'}));
+```
+##### Result
+![Example result](http://i.imgur.com/plQAN2Q.png)
+
+</details>
+
 ## Dependencies
 
 - [tinygradient](https://github.com/mistic100/tinygradient) - Generate gradients