Update Developer Guide (#1210)

Author: ota-meshi

Diff for: README.md

@@ -37,9 +37,12 @@ See [The ESLint Vue Plugin Developer Guide](https://eslint.vuejs.org/developer-g
 
 Before you start writing a new rule, please read [the official ESLint guide](https://eslint.org/docs/developer-guide/working-with-rules).
 
-Next, in order to get an idea how does the AST of the code that you want to check looks like, use one of the following applications:
-- [astexplorer.net](https://astexplorer.net/) - the best tool to inspect ASTs, but it doesn't support Vue template yet
-- [ast.js.org](https://ast.js.org/) - not fully featured, but supports Vue template syntax
+Next, in order to get an idea how does the AST of the code that you want to check looks like, use the [astexplorer.net].
+The [astexplorer.net] is a great tool to inspect ASTs, also Vue templates are supported.
+
+After opening [astexplorer.net], select `Vue` as the syntax and `vue-eslint-parser` as the parser.
+
+[astexplorer.net]: https://astexplorer.net/
 
 Since single file components in Vue are not plain JavaScript, the default parser couldn't be used, so a new one was introduced. `vue-eslint-parser` generates enhanced AST with nodes that represent specific parts of the template syntax, as well as what's inside the `<script>` tag.
 

Diff for: docs/developer-guide/README.md

@@ -28,9 +28,12 @@ We're more than happy to see potential contributions, so don't hesitate. If you
 
 Before you start writing new rule, please read the [official ESLint guide](https://eslint.org/docs/developer-guide/working-with-rules).
 
-Next in order to get an idea how does the AST of the code that you want to check looks like, you can use one of the following applications:
-- [astexplorer.net](https://astexplorer.net/) - best tool to inspect ASTs, but it doesn't support Vue templates yet
-- [ast.js.org](https://ast.js.org/) - not fully featured, but supports Vue templates syntax
+Next, in order to get an idea how does the AST of the code that you want to check looks like, use the [astexplorer.net].
+The [astexplorer.net] is a great tool to inspect ASTs, also Vue templates are supported.
+
+After opening [astexplorer.net], select `Vue` as the syntax and `vue-eslint-parser` as the parser.
+
+[astexplorer.net]: https://astexplorer.net/
 
 Since single file components in Vue are not plain JavaScript, we can't use the default parser, and we had to introduce additional one: `vue-eslint-parser`, that generates enhanced AST with nodes that represent specific parts of the template syntax, as well as what's inside the `<script>` tag.