Add rule docs

docs/rules/undef-init.md

@@ -1,7 +1,7 @@
 # Always initialize variables during declaration. Set it explicitly to undefined, if required. (undef-init)
 
-Please describe the origin of the rule here.
-
+### It is better to explicitly initialize all variable declarations.
+Instead of naked var / let declarations (which are implicitly set to `undefined` anyway), always explicitly set them to `undefined`. It makes clear that you explicitly intended to set them to undefined, not just because you forgot to initialize.
 
 ## Rule Details
 
@@ -11,7 +11,13 @@ Examples of **incorrect** code for this rule:
 
 ```js
 
-// fill me in
+var a;
+let b;
+let c, d;
+let d, e = 1;
+
+for (var f of {}){var g;}
+for (var i in []){var j;}
 
 ```
 
@@ -19,18 +25,25 @@ Examples of **correct** code for this rule:
 
 ```js
 
-// fill me in
+var a = undefined;
+let b = undefined;
+let b1 = b;
+let c = undefined, d = undefined;
+
+for (var f of {}){var g = undefined;}
+
+for (var i in []){do_something()}
 
 ```
 
 ### Options
 
-If there are any options, describe them here. Otherwise, delete this section.
+There are no options
 
 ## When Not To Use It
 
-Give a short description of when it would be appropriate to turn off this rule.
+If you feel setting vraiables implicitly to `undefined` is better than setting them explicitly, then don't use this - use `no-undef-init`.
 
-## Further Reading
+## More
 
-If there are other links that describe the issue this rule addresses, please include them here in a bulleted list.
+Ideally this would be a configuration in the in-built eslint rule `no-undef-init` instead of a separate rule, but wasn't accepted by maintainers. Hence, it's an external plugin.