Update README.

Author: samhocevar

README.md

@@ -1,7 +1,7 @@
 # z8lua
 
-This is a fork of Lua that implements the PICO-8 dialect and adds useful features for
-emulator implementations.
+This is a fork of Lua that implements the [PICO-8](https://www.lexaloffle.com/pico-8.php)
+dialect and adds useful features for emulator implementations.
 
 ### Branches
 
@@ -10,13 +10,14 @@ try to submit patches and PRs against the corresponding feature branch instead o
 
 There are three main feature branches:
 
- - `pico8`: the PICO-8 syntax branch.
+ - `pico8`: this is a “clean” branch that only implements the PICO-8 syntax and type system,
+   with no extra fancy features; a good start if you’re writing your own emulator.
  - `eris`: a branch imported from the [eris persistence patch](https://github.com/fnuecke/eris)
    with bug fixes and improvements ([my pull requests have received little attention so
    far](https://github.com/fnuecke/eris/pulls)); this library provides serialisable snapshots
    of the Lua state (quite useful for emulators).
- - `oua`: allow to switch on-the-fly to zero-based indices in Lua, using the `base(0)` function
-   and back with `base(1)`. I call this the **Oua** language.
+ - `oua`: experimental branch allowing to switch on-the-fly to zero-based indices in Lua, using
+   the `base(0)` function and back with `base(1)`. I call this the **Oua** language.
 
 ## PICO-8 features
 
@@ -30,6 +31,16 @@ There are three main feature branches:
  - binary literals: `0b1001001.10010`
  - works in Windows, Linux, OS X, and many embedded systems
 
+### Limitations
+
+ - Lua functions that rely on the PICO-8 state, particularly the VM memory, are beyond the scope
+   of this software; for a more complete PICO-8 implementation, see the [zepto8
+   emulator](https://github.com/samhocevar/zepto8) which is based on z8lua. The only exceptions
+   are the `@`, `%` and `$` operators (see next section).
+ - the `sqrt`, `sin`, `cos` and `atan2` functions are implemented using floating point, which
+   is inelegant and a potential performance issue, but which also means the results are not
+   bit-by-bit equivalent to the original PICO-8.
+
 ## API extensions
 
 ```c