Adding help for how query strings could be handled

This is the suggested documentation update that came as part of issue pillarjs#209.

Readme.md

@@ -45,7 +45,7 @@ const regexp = pathToRegexp("/foo/:bar", keys);
 // keys = [{ name: 'bar', prefix: '/', delimiter: '/', optional: false, repeat: false, pattern: '[^\\/]+?' }]
 ```
 
-**Please note:** The `RegExp` returned by `path-to-regexp` is intended for ordered data (e.g. pathnames, hostnames). It can not handle arbitrarily ordered data (e.g. query strings, URL fragments, JSON, etc).
+**Please note:** The `RegExp` returned by `path-to-regexp` is intended for ordered data (e.g. pathnames, hostnames). It can not handle arbitrarily ordered data (e.g. query strings, URL fragments, JSON, etc). When using paths that contain query strings, you need to escape the question mark (`?`) to ensure it does not flag a parameter as [optional](#optional).
 
 ### Parameters
 
@@ -138,6 +138,20 @@ regexp.exec("/test/route");
 
 **Tip:** The prefix is also optional, escape the prefix `\/` to make it required.
 
+When dealing with query strings, escape the question mark (`?`) to ensure it doesn't mark a parameter as optional. Handling unordered query string parameters is outside the scope of this library.
+
+```js
+// Example that uses a query string
+const regexp = pathToRegexp("/search/:tableName\\?useIndex=true&term=amazing");
+
+regexp.exec("/search/people?useIndex=true&term=amazing");
+//=> [ '/search/people?useIndex=true&term=amazing', 'people', index: 0, input: '/search/people?useIndex=true&term=amazing', groups: undefined ]
+
+// This library does not handle query strings in different orders
+regexp.exec("/search/people?term=amazing&useIndex=true");
+//=> null
+```
+
 ##### Zero or more
 
 Parameters can be suffixed with an asterisk (`*`) to denote a zero or more parameter matches.