Smaller headings in README.

Author: neelance

README.md

@@ -1,12 +1,10 @@
 GopherJS - A transpiler from Go to JavaScript
-=============================================
+---------------------------------------------
 
-What is supported?
-------------------
+### What is supported?
 GopherJS is able to turn itself (and all packages it uses) into pure JavaScript code that runs in all major browsers. This suggests a quite good coverage of Go's specification. However, there are some known exceptions listed below and some unknown exceptions that I would love to hear about when you find some.
 
-Not yet supported
------------------
+### Not yet supported
 Those features are not implemented yet, but on the roadmap:
 
 - implicit panics (division by zero, etc.)
@@ -15,13 +13,11 @@ Those features are not implemented yet, but on the roadmap:
 - goroutines, channels, select
 - goto
 
-Derivations from Go specification
----------------------------------
+### Derivations from Go specification
 Some tradeoffs have to be made in order to avoid huge performance impacts. Please get in contact if those are deal breakers for you.
 
 - int32, uint32, int64 and uint64 have emulated overflow, all other integer types do not
 - calls on nil cause a panic except for slice types
 
-Interface to external JavaScript
---------------------------------
+### Interface to external JavaScript
 A function's body can be written in JavaScript by putting the code in a string constant with the name `js_[function name]` for package functions and `js_[type name]_[method name]` for methods. In that case, GopherJS disregards the Go function body and instead generates `function(...) { [constant's value] }`. This allows functions to have a Go signature that the type checker can use while being able to call external JavaScript functions.