Create initial readme

Work is still needed on documenting creating your own serdes

README.md

@@ -1,4 +1,118 @@
 # coffi
+[![cljdoc badge](https://cljdoc.org/badge/org.suskalo/coffi)](https://cljdoc.org/d/org.suskalo/coffi/CURRENT)
+[![Clojars Project](https://img.shields.io/clojars/v/org.suskalo/coffi.svg)](https://clojars.org/org.suskalo/coffi)
+
+Coffi is a foreign function interface library for Clojure, using the new
+[Project Panama](https://openjdk.java.net/projects/panama/) that's a part of the
+incubator in Java 17. This allows calling native code directly from Clojure
+without the need for either Java or native code specific to the library, as e.g.
+the JNI does. Coffi focuses on ease of use, including functions and macros for
+creating wrappers to allow the resulting native functions to act just like
+Clojure ones, however this doesn't remove the ability to write systems which
+minimize the cost of marshaling data and optimize for performance, to make use
+of the low-level access Panama gives us.
+
+## Installation
+This library is available on Clojars. Add the following entry to the `:deps` key
+of your `deps.edn`:
+
+```clojure
+org.suskalo/coffi {:mvn/version "0.1.0"}
+```
+
+## Usage
+There are two major components to coffi and interacting with native code:
+manipulating off-heap memory, and loading native code for use with Clojure.
+
+In the simplest cases, the native functions you call will work exclusively with
+built-in types, for example the function `strlen` from libc.
+
+```clojure
+(require '[coffi.ffi :as ffi :refer [defcfn defcstruct]])
+
+(defcfn strlen
+  "Given a string, measures its length in bytes."
+  strlen [::ffi/c-string] ::ffi/long)
+
+(strlen "hello")
+;; => 5
+```
+
+The first argument to `defcfn` is the name of the Clojure var that will hold the
+native function reference, followed by an optional docstring and attribute map,
+then the C function identifier, including the name of the native symbol, a
+vector of argument types, and the return type.
+
+Coffi defines a basic set of primitive types:
+- byte
+- short
+- int
+- long
+- long-long
+- char
+- float
+- double
+- pointer
+
+Each of these types maps to their C counterpart. Values of any of these
+primitive types except for `pointer` will be cast with their corresponding
+Clojure function (with `long-long` mapping to the `long` function) when they are
+passed as arguments to native functions. Additionally, the `c-string` type is
+defined, although it is not primitive.
+
+In addition, some compound types are also defined in coffi, including struct and
+union types. For an example c struct and function:
+
+```c
+typedef struct point {
+    float x;
+    float y;
+} Point;
+
+Point zero(void) {
+    Point res = {};
+
+    res.x = 0.0;
+    res.y = 0.0;
+
+    return res;
+}
+```
+
+The corresponding coffi definition is like so:
+
+```clojure
+(defcfn zero-point
+  "zero" [] [::ffi/struct [[:x ::ffi/float] [:y ::ffi/float]]])
+
+(zero-point)
+;; => {:x 0.0,
+;;     :y 0.0}
+```
+
+Writing out struct definitions like this every time would get tedious, so the
+macro `defalias` is used to define a struct alias.
+
+```clojure
+(defalias ::point
+  [::ffi/struct
+   [:x ::ffi/float]
+   [:y ::ffi/float]])
+
+(defcfn zero-point
+  "zero" [] ::point)
+```
+
+In addition to structs, there is also support for passing Clojure functions as
+callbacks to native functions, as well as calling function pointers returned
+from native functions as Clojure functions.
+
+```clojure
+(defcfn higher-order
+  "higher_order" [[::ffi/fn [::ffi/c-string] ::ffi/int]] ::ffi/int)
+```
+
+TODO Talk about writing your own serdes
 
 ## License