Reverse rule on string concatenation for long lines (airbnb#995)

Broken and concatenated long strings are painful to work with and
produce less readable and searchable code. I think we should reverse
this rule.

Unfortunately, the max-len rule currently does not allow for this, but
there is currently a proposal to add an option to ESLint's max-len rule
that would allow for strings to be ignored.

  eslint/eslint#5805

There have also been discussions around performance of string
concatenation (airbnb#40), but I
don't think that is very relevant here so I removed the links to them.

README.md

@@ -483,25 +483,24 @@ Other Style Guides
     ```
 
   <a name="strings--line-length"></a><a name="6.2"></a>
-  - [6.2](#strings--line-length) Strings that cause the line to go over 100 characters should be written across multiple lines using string concatenation.
+  - [6.2](#strings--line-length) Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.
 
-  <a name="strings--concat-perf"></a><a name="6.3"></a>
-  - [6.3](#strings--concat-perf) Note: If overused, long strings with concatenation could impact performance. [jsPerf](http://jsperf.com/ya-string-concat) & [Discussion](https://github.com/airbnb/javascript/issues/40).
+    > Why? Broken strings are painful to work with and make code less searchable.
 
     ```javascript
-    // bad
-    const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';
-
     // bad
     const errorMessage = 'This is a super long error that was thrown because \
     of Batman. When you stop to think about how Batman had anything to do \
     with this, you would get nowhere \
     fast.';
 
-    // good
+    // bad
     const errorMessage = 'This is a super long error that was thrown because ' +
       'of Batman. When you stop to think about how Batman had anything to do ' +
       'with this, you would get nowhere fast.';
+
+    // good
+    const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';
     ```
 
   <a name="es6-template-literals"></a><a name="6.4"></a>