docs: document Error comparison rules (#42)

* Document `Error` comparison rules

Closes #40

Author: shvaikalesh

README.md

@@ -97,7 +97,7 @@ The primary export of `deep-eql` is function that can be given two objects to co
 
 ### Rules
 
-- Strict equality for non-traversable nodes according to [`Object.is`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is).
+- Strict equality for non-traversable nodes according to [`Object.is`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is):
   - `eql(NaN, NaN).should.be.true;`
   - `eql(-0, +0).should.be.false;`
 - All own and inherited enumerable properties are considered:
@@ -106,3 +106,6 @@ The primary export of `deep-eql` is function that can be given two objects to co
 - Arguments are not Arrays:
   - `eql([], arguments).should.be.false;`
   - `eql([], Array.prototype.slice.call(arguments)).should.be.true;`
+- Error objects are compared by reference (see https://github.com/chaijs/chai/issues/608):
+  - `eql(new Error('msg'), new Error('msg')).should.be.false;`
+  - `var err = new Error('msg'); eql(err, err).should.be.true;`