Adds a note about LuaJIT behaviour (#6)

Clarifies that LuaJIT behaves the same as Lua 5.2 in that the
debug.traceback replacement trick does not work. You need to
wrap the main chunk in a xpcall call.

fixes #6

diff --git a/README.md b/README.md
index 4e08be9..569f1f6 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@

 StackTracePlus provides enhanced stack traces for [Lua 5.1, Lua 5.2][1] and [LuaJIT][2].

-StackTracePlus can be used as a replacement for debug.traceback. It gives detailed information about locals, tries to guess
+StackTracePlus can be used as a replacement for debug.traceback. It gives detailed information about locals, tries to guess
 function names when they're not available, etc, so, instead of

     lua5.1.exe: D:\trunk_git\sources\stacktraceplus\test\test.lua:10: attempt to concatenate a nil value
@@ -34,8 +34,8 @@ you'll get

 ## Usage #

-StackTracePlus can be used as a replacement for `debug.traceback`, as an `xpcall` error handler or even from C code. Note that
-only the Lua 5.1 interpreter allows the traceback function to be replaced "on the fly". Lua 5.2 always calls luaL_traceback.
+StackTracePlus can be used as a replacement for `debug.traceback`, as an `xpcall` error handler or even from C code. Note that
+only the Lua 5.1 interpreter allows the traceback function to be replaced "on the fly". LuaJIT and Lua 5.2 always calls luaL_traceback internally so there is no easy way to override that.

 ```lua
 local STP = require "StackTracePlus"
@@ -53,7 +53,7 @@ end
 test()
 ```

-That script will output:
+That script will output (only with Lua 5.1):

     lua5.1: example.lua:11: an error
     Stack Traceback
@@ -70,7 +70,7 @@ That script will output:
     (5) main chunk of file 'example.lua' at line 14
     (6)  C function 'function: 00637B30'

-**StackTracePlus** is aware of the usual Lua libraries, like *coroutine*, *table*, *string*, *io*, etc and functions like
+**StackTracePlus** is aware of the usual Lua libraries, like *coroutine*, *table*, *string*, *io*, etc and functions like
 *print*, *pcall*, *assert*, and so on.

 You can also make STP aware of your own tables and functions by calling *add_known_function* and *add_known_table*.