Docs: Minor tweaks to working with rules page (#9824)

docs/developer-guide/working-with-rules.md

@@ -2,10 +2,11 @@
 
 **Note:** This page covers the most recent rule format for ESLint >= 3.0.0. There is also a [deprecated rule format](./working-with-rules-deprecated.md).
 
-Each rule in ESLint has two files named with its identifier (for example, `no-extra-semi`).
+Each rule in ESLint has three files named with its identifier (for example, `no-extra-semi`).
 
 * in the `lib/rules` directory: a source file (for example, `no-extra-semi.js`)
 * in the `tests/lib/rules` directory: a test file (for example, `no-extra-semi.js`)
+* in the `docs/rules` directory: a Markdown documentation file (for example, `no-extra-semi.md`)
 
 **Important:** If you submit a **core** rule to the ESLint repository, you **must** follow some conventions explained below.
 
@@ -28,7 +29,8 @@ module.exports = {
         docs: {
             description: "disallow unnecessary semicolons",
             category: "Possible Errors",
-            recommended: true
+            recommended: true,
+            url: "https://eslint.org/docs/rules/no-extra-semi"
         },
         fixable: "code",
         schema: [] // no options
@@ -52,6 +54,7 @@ The source file for a rule exports an object with the following properties.
     * `description` (string) provides the short description of the rule in the [rules index](../rules/)
     * `category` (string) specifies the heading under which the rule is listed in the [rules index](../rules/)
     * `recommended` (boolean) is whether the `"extends": "eslint:recommended"` property in a [configuration file](../user-guide/configuring.md#extending-configuration-files) enables the rule
+    * `url` (string) specifies the URL at which the full documentation can be accessed
 
     In a custom rule or plugin, you can omit `docs` or include any properties that you need in it.