docs(readme): use .md extension, add badges

Signed-off-by: Frazer Smith <[email protected]>

readme.markdown → README.md

@@ -1,17 +1,26 @@
 # safe-regex2
 
-detect potentially
+[![CI](https://github.com/fastify/safe-regex2/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/fastify/safe-regex2/actions/workflows/ci.yml)
+[![NPM version](https://img.shields.io/npm/v/safe-regex2.svg?style=flat)](https://www.npmjs.com/package/safe-regex2)
+[![neostandard javascript style](https://img.shields.io/badge/code_style-neostandard-brightgreen?style=flat)](https://github.com/neostandard/neostandard)
+
+Detect potentially
 [catastrophic](http://regular-expressions.mobi/catastrophic.html)
 [exponential-time](http://perlgeek.de/blog-en/perl-tips/in-search-of-an-exponetial-regexp.html)
 regular expressions by limiting the
-[star height](https://en.wikipedia.org/wiki/Star_height) to 1
+[star height](https://en.wikipedia.org/wiki/Star_height) to 1.
 
 This is a fork of https://github.com/substack/safe-regex at 1.1.0.
 
 WARNING: This module has both false positives and false negatives.
 It is not meant as a full checker, but it detect basic cases.
 
-# example
+## Install
+```
+npm i safe-regex2
+```
+
+## Example
 
 ``` js
 const safe = require('safe-regex2');
@@ -30,13 +39,13 @@ $ node safe.js '\blocation\s*:[^:\n]+\b(Oakland|San Francisco)\b'
 true
 ```
 
-# methods
+## Methods
 
 ``` js
 const safe = require('safe-regex')
 ```
 
-## const ok = safe(re, opts={})
+### const ok = safe(re, opts={})
 
 Return a boolean `ok` whether or not the regex `re` is safe and not possibly
 catastrophic.
@@ -48,14 +57,6 @@ If the `re` is a string and is an invalid regex, returns `false`.
 * `opts.limit` - maximum number of allowed repetitions in the entire regex.
 Default: `25`.
 
-# install
-
-With [npm](https://npmjs.org) do:
-
-```
-npm install safe-regex2
-```
-
-# license
+## License
 
-MIT
+Licensed under [MIT](./LICENSE).