Add rule with docs

docs/rules/max-dependencies.md

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+# max-dependencies
+
+Forbid modules to have too many dependencies (`import` or `require` statements).
+
+This is a useful rule because a module with too many dependencies is code smell, and usually indicates the module is doing too much and/or should be broken up into smaller modules.
+
+Importing multiple named exports from a single module will only count once (e.g. `import {x, y, z} from './foo'` will only count as a single dependency).
+
+### Options
+
+This rule takes the following option:
+
+`max`: The maximum number of dependencies allowed. Anything over will trigger the rule. **Default is 10** if the rule is enabled and no `max` is specified.
+
+You can set the option like this:
+
+```js
+"import/max-dependencies": ["error", {"max": 10}]
+```
+
+
+## Example
+
+Given a max value of `{"max": 2}`:
+
+### Fail
+
+```js
+import a from './a'; // 1
+const b = require('./b'); // 2
+import c from './c'; // 3 - exceeds max!
+```
+
+### Pass
+
+```js
+import a from './a'; // 1
+const anotherA = require('./a'); // still 1
+import {x, y, z} from './foo'; // 2
+```
+
+## When Not To Use It
+
+If you don't care how many dependencies a module has.

src/rules/max-dependencies.js

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+import Set from 'es6-set'
+import isStaticRequire from '../core/staticRequire'
+
+const DEFAULT_MAX = 10
+
+const countDependencies = (dependencies, lastNode, context) => {
+  const {max} = context.options[0] || { max: DEFAULT_MAX }
+
+  if (dependencies.size > max) {
+    context.report(
+      lastNode,
+      `Maximum number of dependencies (${max}) exceeded.`
+    )
+  }
+}
+
+module.exports = context => {
+  const dependencies = new Set() // keep track of dependencies
+  let lastNode // keep track of the last node to report on
+
+  return {
+    ImportDeclaration(node) {
+      dependencies.add(node.source.value)
+      lastNode = node.source
+    },
+
+    CallExpression(node) {
+      if (isStaticRequire(node)) {
+        const [ requirePath ] = node.arguments
+        dependencies.add(requirePath.value)
+        lastNode = node
+      }
+    },
+
+    'Program:exit': function () {
+      countDependencies(dependencies, lastNode, context)
+    },
+  }
+}
+
+module.exports.schema = [
+  {
+    'type': 'object',
+    'properties': {
+      'max': { 'type': 'number' },
+    },
+    'additionalProperties': false,
+  },
+]