[PATCH] Document khash

@@ -0,0 +1,109 @@
+khash
+=====
+
+The khash API is a collection of macros that instantiate hash table
+routines for arbitrary key/value types.
+
+It lives in 'khash.h', which we imported from
+  https://github.com/attractivechaos/klib/blob/master/khash.h
+and then gitified somewhat.
+
+Usage
+-----
+
+------------
+#import "khash.h"
+KHASH_INIT(NAME, key_t, value_t, is_map, key_hash_fn, key_equal_fn)
+------------
+
+The arguments are as follows:
+
+`NAME`::
+	Used to give your hash and its API a unique name.  Spelled
+	`NAME` here to set it apart from the rest of the names, but
+	generally should be a lowercase C identifier.
+
+`key_t`::
+	Type of the keys for your hashes.  Generally should be
+	`const`.
+
+`value_t`::
+	Type of the values of your hashes.
+
+`is_map`::
+	Whether the hash holds values.  Set to 0 to create a hash for
+	use as a set.
+
+`khint_t key_hash_fn(key_t)`::
+	Hash function.
+
+`int key_equal_fn(key_t a, key_t b)`::
+	Comparison function.  Return 1 if the two keys are the same, 0
+	otherwise.
+
+These two functions may also be macros.
+
+
+API
+---
+
+The above instantiation defines a single type:
+
+`kh_NAME_t`::
+	A struct that holds your hash.  You should only ever use
+	pointers to it.
+
+After the above instantiation, the following functions and
+function-like macros are defined:
+
+`kh_NAME_t *kh_init_NAME(void)`::
+	Allocate and initialize a new hash table.
+
+`void kh_destroy_NAME(kh_NAME_t *)`::
+	Free the hash table.
+
+`void kh_clear_NAME(kh_NAME_t *)`::
+	Clear (but do not free) the hash table.
+
+`khint_t kh_get_NAME(const kh_NAME_t *hash, key_t key)`::
+	Find the given key in the hash table.  The returned khint_t
+	should be treated as an opaque iterator token that indexes
+	into the hash.  Use `kh_value` to get the value from the
+	token.  If the key does not exist, returns kh_end(hash).
+
+`khint_t kh_put_NAME(const kh_NAME_t *hash, key_t key, int *ret)`::
+	Put the given key in the hash table.  The returned khint_t
+	should be treated as an opaque iterator token that indexes
+	into the hash.  Use `kh_value(hash, token) = value` to assign
+	a value after putting the key.
++
+'ret' tells you whether the key already existed: it is -1 if the
+operation failed; 0 if the key was already present; 1 if the bucket is
+empty; 2 if the bucket has been deleted.
+
+`kh_key(kh_NAME_t *, khint_t)`::
+	The key slot for the given iterator token.  This can be used
+	as an lvalue, but you must not change the key!
+
+`kh_value(kh_NAME_t *, khint_t)`::
+`kh_val(kh_NAME_t *, khint_t)`::
+	The value slot for the given iterator token.  This can be used
+	as an lvalue.
+
+`int kh_exist(const kh_NAME_t *, key_t)`::
+	Tests if the given bucket contains data.
+
+`khint_t kh_begin(const kh_NAME_t *)`::
+	The smallest iterator token.
+
+`khint_t kh_end(const kh_NAME_t *)`::
+	The beyond-the-hash iterator token; it is one larger than the
+	largest token that points to a hash member.
+
+`khint_t kh_size(const kh_NAME_t *)`::
+	The number of elements in the hash.
+
+`kh_foreach(const kh_NAME_t *, keyvar, valuevar, { ... loop body ... })`::
+	Iterate over every key-value pair in the hash.  This is a
+	macro, and keyvar and valuevar must be pre-declared of type
+	key_t and value_t, respectively.