Fix: Add max-classes-per-file to main rules directory

This fixes a broken link in docs/rules index. Root cause is eslint/eslint-release#22

docs/rules/max-classes-per-file.md

@@ -0,0 +1,65 @@
+---
+title: max-classes-per-file - Rules
+layout: doc
+edit_link: https://github.com/eslint/eslint/edit/master/docs/rules/max-classes-per-file.md
+---
+<!-- Note: No pull requests accepted for this file. See README.md in the root directory for details. -->
+
+# enforce a maximum number of classes per file (max-classes-per-file)
+
+Files containing multiple classes can often result in a less navigable
+and poorly structured codebase. Best practice is to keep each file
+limited to a single responsibility.
+
+## Rule Details
+
+This rule enforces that each file may contain only a particular number
+of classes and no more.
+
+Examples of **incorrect** code for this rule:
+
+```js
+/*eslint max-classes-per-file: "error"*/
+
+class Foo {}
+class Bar {}
+```
+
+Examples of **correct** code for this rule:
+
+```js
+/*eslint max-classes-per-file: "error"*/
+
+class Foo {}
+```
+
+## Options
+
+This rule has a numeric option (defaulted to 1) to specify the
+maximum number of classes.
+
+For example:
+
+```json
+{
+    "max-classes-per-file": ["error", 1]
+}
+```
+
+Examples of **correct** code for this rule with the numeric option set to `2`:
+
+```js
+/* eslint max-classes-per-file: ["error", 2] */
+
+class Foo {}
+class Bar {}
+```
+
+## Version
+
+This rule was introduced in ESLint 5.0.0-alpha.3.
+
+## Resources
+
+* [Rule source](https://github.com/eslint/eslint/tree/master/lib/rules/max-classes-per-file.js)
+* [Documentation source](https://github.com/eslint/eslint/tree/master/docs/rules/max-classes-per-file.md)