docs: include details about fixture opts

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 When filing bugs, if it's an inlining issue, ensure you include full test source to work against.
 
-This can be via a [PR](https://github.com/remy/inliner/pulls) or by linking to a [gist](https://gist.github.com) that include at least **two** files:
+This can be via a [PR](https://github.com/remy/inliner/pulls) or by linking to a [gist](https://gist.github.com) that include at least **two** (first) files:
 
 - `<issue>.src.html`
 - `<issue>.result.html`
@@ -11,6 +11,8 @@ When these are put in the [fixtures](https://github.com/remy/inliner/tree/master
 
 If there are any external assets the example needs, please also include these and name them with the same root as your example, i.e. `<issue>.css` or `<issue>.min.js` etc.
 
+In addition `<issue>.opt.json` can be loaded to help specify runtime options during the test. 
+
 **To test a single fixture you can use: `npm test <issue>`**
 
 ## Commit messages
@@ -29,4 +31,4 @@ This will allow for the automatic changelog to generate correctly.
 
 ## Code standards
 
-Ensure that your code adheres to the included `.jshintrc` and `.jscsrc` configs.
+Ensure that your code adheres to the included `.jshintrc` and `.jscsrc` configs.