Create COMPATIBILITY.md (sinonjs#2051)

* Create COMPATIBILITY.md
* Update README.md

Co-Authored-By: Carl-Erik Kopseng <carlerik@gmail.com>

COMPATIBILITY.md

@@ -0,0 +1,27 @@
+# Compatibility
+
+This file is the canonical resource for compatibility goals for the libraries that make up Sinon.JS.
+
+## ES5.1
+
+The source is written as [ES5.1][ES5] and requires no transpiler or polyfills.
+
+Sinon.JS uses feature detection to support [ES6][ES6] features, but does not rely on any of the new syntax introduced in [ES6][ES6] and remains compatible with [ES5.1][ES5] runtimes.
+
+## Runtimes
+
+Sinon.JS aims at supporting the following runtimes:
+
+* Firefox 45+
+* Chrome 48+
+* Internet Explorer 11+
+* Edge 14+
+* Safari 9+
+* Node LTS versions
+
+<p align=center>
+<a href="https://saucelabs.com/u/sinonjs"><img src="https://saucelabs.com/browser-matrix/sinonjs.svg" alt="Sauce Test Status"></a>
+</p>
+
+[ES5]: http://www.ecma-international.org/ecma-262/5.1/
+[ES6]: http://www.ecma-international.org/ecma-262/6.0/

CONTRIBUTING.md

@@ -65,23 +65,7 @@ This will also install a pre-commit hook, that runs style validation on staged f
 
 ### Compatibility
 
-#### ES5.1
-
-Sinon's source is written as [ES5.1][ES5] and requires no transpiler or polyfills.
-
-Sinon.JS uses feature detection to support [ES6][ES6] features, but does not rely on any of the new syntax introduced in [ES6][ES6] and remains compatible with [ES5.1][ES5] runtimes.
-
-#### Runtimes
-
-Sinon.JS aims at supporting the following runtimes:
-
-* Firefox 45+
-* Chrome 48+
-* Internet Explorer 11+
-* Edge 14+
-* Safari 9+
-* Node LTS versions
-
+Please see [`COMPATIBILITY.md`][COMPATIBILITY.md]
 
 ### Linting and style
 
@@ -137,5 +121,3 @@ To build simply run
     $ node build.js
 
 
-[ES5]: http://www.ecma-international.org/ecma-262/5.1/
-[ES6]: http://www.ecma-international.org/ecma-262/6.0/

README.md

@@ -26,6 +26,10 @@
 <a href="https://saucelabs.com/u/sinonjs"><img src="https://saucelabs.com/browser-matrix/sinonjs.svg" alt="Sauce Test Status"></a>
 </p>
 
+## Compatibility
+
+For details on compatibility and browser support, please see [`COMPATIBILITY.md`][COMPATIBILITY.md]
+
 ## Installation
 
 via [npm](https://github.com/npm/npm)