[functions] add source about func declarations & blocks in different browsers

Author: hshoff

README.md

@@ -255,6 +255,7 @@
     ```
 
   - Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears.
+  - **Note:** ECMA-262 defines a `block` as a list of statements. A function declartion is not a statement. (Read ECMA-262's note on this issue)[http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf#page=97].
 
     ```javascript
     // bad