Clean up readme a little (#33)

* Remove Contributing section from readme

Most of the Chrome audit rules that make sense for this plugin have been
implemented, and once we remove that sentence there's not really much
left of this section so I ended up just removing it all.

* Enable syntax highlighting for *sh code in readme

* Use 2 spaces for indentation in JSON examples in readme

2 spaces seems pretty standard and also matches the indentation style of
this project.

* Sort list of rules alphabetically

Having these rules in alphabetical order will make it easier to find
what you are looking for and for future developers to know where to put
new things.

* Improve rule descriptions in readme

This mostly adds backticks around things that represent code.

Author: lencioni

README.md

@@ -33,13 +33,13 @@ Ryan Florence built out this awesome runtime-analysis tool called [react-a11y](h
 
 You'll first need to install [ESLint](http://eslint.org):
 
-```
+```sh
 $ npm i eslint --save-dev
 ```
 
 Next, install `eslint-plugin-jsx-a11y`:
 
-```
+```sh
 $ npm install eslint-plugin-jsx-a11y --save-dev
 ```
 
@@ -51,9 +51,9 @@ Add `jsx-a11y` to the plugins section of your `.eslintrc` configuration file. Yo
 
 ```json
 {
-    "plugins": [
-        "jsx-a11y"
-    ]
+  "plugins": [
+    "jsx-a11y"
+  ]
 }
 ```
 
@@ -62,32 +62,29 @@ Then configure the rules you want to use under the rules section.
 
 ```json
 {
-    "rules": {
-        "jsx-a11y/rule-name": 2
-    }
+  "rules": {
+    "jsx-a11y/rule-name": 2
+  }
 }
 ```
 
 ## Supported Rules
 
-- [img-uses-alt](docs/rules/img-uses-alt.md): Enforce that img jsx elements use the alt attribute.
-- [onclick-uses-role](docs/rules/onclick-uses-role.md): Enforce that non-interactive, visible elements (such as div) that have click handlers use the role attribute.
-- [mouse-events-map-to-key-events](docs/rules/mouse-events-map-to-key-events.md): Enforce that onMouseOver/onMouseOut are accompanied by onFocus/onBlur for strictly keyboard users.
-- [use-onblur-not-onchange](docs/rules/use-onblur-not-onchange.md): Enforce that onBlur is used instead of onChange.
-- [no-access-key](docs/rules/no-access-key.md): Enforce that the accessKey prop is not used on any element to avoid complications with keyboard commands used by a screenreader.
-- [label-uses-for](docs/rules/label-uses-for.md): Enforce that label elements have the htmlFor attribute
-- [redundant-alt](docs/rules/redundant-alt.md): Enforce img alt attribute does not contain the word image, picture, or photo.
-- [no-hash-href](docs/rules/no-hash-href.md): Enforce an anchor element's href prop value is not just #.
-- [no-invalid-aria](docs/rules/no-invalid-aria.md): Enforce all aria-* properties are valid.
-- [valid-aria-role](docs/rules/valid-aria-role.md): Enforce that elements with ARIA roles must use a valid, non-abstract ARIA role.
-- [valid-aria-proptype](docs/rules/valid-aria-proptype.md): Enforce ARIA state and property values are valid.
+- [avoid-positive-tabindex](docs/rules/avoid-positive-tabindex.md): Enforce `tabIndex` value is not greater than zero.
+- [img-uses-alt](docs/rules/img-uses-alt.md): Enforce that `<img>` JSX elements use the `alt` prop.
+- [label-uses-for](docs/rules/label-uses-for.md): Enforce that `<label>` elements have the `htmlFor` prop.
+- [mouse-events-map-to-key-events](docs/rules/mouse-events-map-to-key-events.md): Enforce that `onMouseOver`/`onMouseOut` are accompanied by `onFocus`/`onBlur` for keyboard-only users.
+- [no-access-key](docs/rules/no-access-key.md): Enforce that the `accessKey` prop is not used on any element to avoid complications with keyboard commands used by a screenreader.
+- [no-hash-href](docs/rules/no-hash-href.md): Enforce an anchor element's `href` prop value is not just `#`.
+- [no-invalid-aria](docs/rules/no-invalid-aria.md): Enforce all `aria-*` props are valid.
+- [no-unsupported-elements-use-aria](docs/rules/no-unsupported-elements-use-aria.md): Enforce that elements that do not support ARIA roles, states, and properties do not have those attributes.
+- [onclick-has-focus](docs/rules/onclick-has-focus.md): Enforce that elements with `onClick` handlers must be focusable.
+- [onclick-uses-role](docs/rules/onclick-uses-role.md): Enforce that non-interactive, visible elements (such as `<div>`) that have click handlers use the role attribute.
+- [redundant-alt](docs/rules/redundant-alt.md): Enforce `<img>` alt prop does not contain the word "image", "picture", or "photo".
 - [role-requires-aria](docs/rules/role-requires-aria.md): Enforce that elements with ARIA roles must have all required attributes for that role.
-- [no-unsupported-elements-use-aria](docs/rules/no-unsupported-elements-use-aria.md): Enforce that elements that do not support ARIA roles, states and properties do not have those attributes.
-- [avoid-positive-tabindex](docs/rules/avoid-positive-tabindex.md): Enforce tabIndex value is not greater than zero.
-- [onclick-has-focus](docs/rules/onclick-has-focus.md): Enforce that elements with onClick handlers must be focusable.
-
-## Contributing
-Feel free to contribute! I am currently using [Google Chrome's Audit Rules](https://github.com/GoogleChrome/accessibility-developer-tools/wiki/Audit-Rules) to map out as rules for this plugin.
+- [use-onblur-not-onchange](docs/rules/use-onblur-not-onchange.md): Enforce that `onBlur` is used instead of `onChange`.
+- [valid-aria-proptype](docs/rules/valid-aria-proptype.md): Enforce ARIA state and property values are valid.
+- [valid-aria-role](docs/rules/valid-aria-role.md): Enforce that elements with ARIA roles must use a valid, non-abstract ARIA role.
 
 ## License