adding text to README

Author: JeffBezanson

Diff for: README.md

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+## What's here
+
+This is a collection of useful public-domain C code by myself and others.
+The best way to use it is to pick and choose the bits you need and
+statically compile them in.
+Take a look at `libsupportinit.c` to see if there are any initialization
+functions you might need to call.
+A summary of the available functionality follows.
+
+### IOS
+
+A replacement for parts of C's stdio library, supporting file-descriptor-backed
+and memory-backed I/O streams with a uniform interface. Also has special
+support for UTF-8, and slightly relaxed semantics that allow keeping data
+buffered longer.
+
+### Bit vectors
+
+This is a pretty thorough bit vector library. The most interesting thing about
+it is that it supports many operations on contiguous sub-vectors of bits.
+This is helpful when implementing strided multi-dimensional arrays of bits.
+
+### Arraylist
+
+A very simple growable array.
+
+### dirpath.c
+
+Contains the function `get_exename`, which can tell you the path to your
+running executable on Linux, Windows, and Mac OS X. Allows you to locate
+application files without "installing" anything.
+
+### dump.c
+
+A routine for printing hex dumps.
+
+### Hashing
+
+High-quality hash functions for strings and integers.
+
+### Hash table
+
+A fast linear-probing hash table. Hardly ever allocates memory when
+inserting a key. It generally avoids collisions by using good hash functions.
+Use it by defining your hash function and equality predicate, then
+invoking a macro. See `ptrhash.c` for an example.
+
+### Other
+
+Some convenient wrapper functions for using sockets and time info on
+multiple platforms.