docs: add middleware guide (#4877)

* adding guide content

* fix link

* CR comments

* fixing a typo

* fixing example

docs/guides/middleware.md

@@ -0,0 +1,74 @@
+# Middleware
+
+Middleware is a Video.js feature that allows interaction with and modification of how the `Player` and `Tech` talk to each other. For more in-depth information, check out our [feature spotlight](http://blog.videojs.com/feature-spotlight-middleware/).
+
+## Table of Contents
+
+* [Understanding Middleware](#understanding-middleware)
+* [Using Middleware](#using-middleware)
+* [setSource](#setsource)
+
+## Understanding Middleware
+
+Middleware are functions that return an object with methods matching those on the `Tech`. There are currently a limited set of allowed methods that will be understood by middleware. These are: `buffered`, `currentTime`, `setCurrentTime`, `duration`, `seekable` and `played`.
+
+These allowed methods are split into two categories: `getters` and `setters`. Setters will be called on the `Player` first and run through middleware(from left to right) before calling the method, with its arguments, on the `Tech`. Getters are called on the `Tech` first and are run though middleware(from right to left) before returning the result to the `Player`.
+
+```
++----------+                      +----------+
+|          |  setter middleware   |          |
+|          +---------------------->          |
+|  Player  |                      |   Tech   |
+|          <----------------------+          |
+|          |  getter middleware   |          |
++----------+                      +----------+
+```
+
+## Using Middleware
+
+Middleware are registered to a video MIME type, and will be run for any source with that type.
+
+```javascript
+videojs.use('video/mp4', myMiddleware);
+```
+
+You can also register a middleware on all sources by registering it on `*`.
+
+```javascript
+videojs.use('*', myMiddleware);
+```
+
+Your middleware should be a function that takes a player as an argument and returns an object with methods on it like below:
+
+```javascript
+var myMiddleware = function(player) {
+  return {
+    currentTime: function(ct) {
+      return ct / 2;
+    },
+    setCurrentTime: function(time) {
+      return time * 2;
+    },
+    ...
+  };
+};
+
+videojs.use('*', myMiddleware);
+```
+
+
+## setSource
+
+`setSource` is a required method for all middleware and must be included in the returned object. If your middlware is not manipulating or rejecting the source, you can pass along the source by doing the following:
+
+```javascript
+videojs.use('*', function(player) {
+  return {
+    setSource: function(srcObj, next) {
+      // pass null as the first argument to indicate that the source is not rejected
+      next(null, srcObj);
+    }
+  };
+});
+```
+

src/js/tech/middleware.js

@@ -76,7 +76,7 @@ function setSourceHelper(src = {}, middleware = [], next, player, acc = [], last
       // we've succeeded, now we need to go deeper
       acc.push(mw);
 
-      // if it's the same time, continue does the current chain
+      // if it's the same type, continue down the current chain
       // otherwise, we want to go down the new chain
       setSourceHelper(_src,
           src.type === _src.type ? mwrest : middlewares[_src.type],