Fix: Add prefer-object-spread to main rules directory

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

docs/rules/prefer-object-spread.md

@@ -0,0 +1,65 @@
+---
+title: prefer-object-spread - Rules
+layout: doc
+edit_link: https://github.com/eslint/eslint/edit/master/docs/rules/prefer-object-spread.md
+---
+<!-- Note: No pull requests accepted for this file. See README.md in the root directory for details. -->
+
+# Prefer use of an object spread over `Object.assign` (prefer-object-spread)
+
+(fixable) The `--fix` option on the [command line](../user-guide/command-line-interface#fix) can automatically fix some of the problems reported by this rule.
+
+When Object.assign is called using an object literal as the first argument, this rule requires using the object spread syntax instead. This rule also warns on cases where an `Object.assign` call is made using a single argument that is an object literal, in this case, the `Object.assign` call is not needed.
+
+## Rule Details
+
+Examples of **incorrect** code for this rule:
+
+```js
+
+Object.assign({}, foo)
+
+Object.assign({}, {foo: 'bar'})
+
+Object.assign({ foo: 'bar'}, baz)
+
+Object.assign({ foo: 'bar' }, Object.assign({ bar: 'foo' }))
+
+Object.assign({}, { foo, bar, baz })
+
+Object.assign({}, { ...baz })
+
+// Object.assign with a single argument that is an object literal
+Object.assign({});
+
+Object.assign({ foo: bar });
+```
+
+Examples of **correct** code for this rule:
+
+```js
+
+Object.assign(...foo);
+
+// Any Object.assign call without an object literal as the first argument
+Object.assign(foo, { bar: baz });
+
+Object.assign(foo, Object.assign(bar));
+
+Object.assign(foo, { bar, baz })
+
+Object.assign(foo, { ...baz });
+```
+
+## When Not To Use It
+
+When you don't care about syntactic sugar added by the object spread property.
+
+## Version
+
+This rule was introduced in ESLint 5.0.0-alpha.3.
+
+## Resources
+
+* [Rule source](https://github.com/eslint/eslint/tree/master/lib/rules/prefer-object-spread.js)
+* [Documentation source](https://github.com/eslint/eslint/tree/master/docs/rules/prefer-object-spread.md)