Add a README.md to explain what is in the repository.

Author: samhocevar

README.md

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+# z8lua
+
+This is a fork of Lua that implements the PICO-8 dialect and adds useful features for
+emulator implementations.
+
+### Branches
+
+The main `zepto8` branch is a composite branch built from several feature branches. Please
+try to submit patches and PRs against the corresponding feature branch instead of `zepto8`.
+
+There are three main feature branches:
+
+ - `pico8`: the PICO-8 syntax branch.
+ - `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.
+
+## PICO-8 features
+
+ - short `if` syntax (on one line)
+ - short `print` syntax (`?` at the beginning of a line)
+ - compound assignment operators: `+=` `/=` etc.
+ - C style not equal operator: `!=`
+ - C++ style comments with `//`
+ - fixed-point arithmetic with overflows, infinity etc.
+ - the PICO-8 math library (`shr`, `atan2`, `flr` etc.)
+ - binary literals: `0b1001001.10010`
+ - works in Windows, Linux, OS X, and many embedded systems
+
+## API extensions
+
+```c
+LUA_API void lua_setpico8memory (lua_State *L, unsigned char const *p);
+```
+
+Provide the Lua VM with a 64-KiB address space for use with the `@`, `%` and `$` operators
+(shorthands for `peek`, `peek2`, and `peek4`). Otherwise these operators will always return 0.