Update readme and changelog re: blend

Author: danburzo

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## Culori Changelog
 
+### 0.8.0
+
+Adds `culori.blend()` ([#59](https://github.com/Evercoder/culori/issues/59)).
+
 ### 0.7.4
 
 Fixes a typo that caused `formatter('rgb')('#f0f0f0f0')` and similar examples to ignore the `alpha` channel of the input color, generating `rgb(...)` instead of `rgba(...)`.

README.md

@@ -432,6 +432,10 @@ Available blending modes:
 -   `difference`
 -   `exclusion`
 
+> **Note:** culori currently implements the _separable_ blend modes, that is the blend modes that work on each channel in the color space independently. _color_, _hue_, _saturation_, and _lightness_ modes are not yet available.
+
+An example of blending three colors:
+
 ```js
 culori.blend(
 	['rgba(255, 0, 0, 0.5)', 'rgba(0, 255, 0, 0.5)', 'rgba(0, 0, 255, 0.5)'],
@@ -440,6 +444,14 @@ culori.blend(
 // => { mode: 'rgb', alpha: 0.875, r: 0.57..., g: 0.57..., b:0.57... }
 ```
 
+In addition to strings, the _type_ parameter supports a _function (b, s) → v_ that takes the values of the _backdrop_ and _source_ color to return the blended value. This allows you to write your own (separable) blending functions. For example, an _average_ blending mode:
+
+```js
+culori.blend(['red', 'green'], function average(b, s) {
+	return (b + s) / 2;
+});
+```
+
 ### Extending culori
 
 <a name="culoriDefineMode" href="#culoriDefineMode">#</a> culori.**defineMode**(_definition_) [<>](https://github.com/evercoder/culori/blob/master/src/modes.js 'Source')