readme: super brief info

ReadMe.md

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+
+# c-capnproto
+
+C support for [Cap'n Proto](http://kentonv.github.io/capnproto), an efficient protocol for sharing data and capabilities.
+
+[![Build Status](https://travis-ci.org/liamstask/c-capnproto.svg?branch=master)](https://travis-ci.org/liamstask/c-capnproto)
+
+## status
+
+This is a fork of [James McKaskill's great work](https://github.com/jmckaskill/c-capnproto), which has been untouched for a while, so I've been bringing it up to date with current cap'n proto progress, and doing some doc/testing work. Should the upstream fork become active again, I'd hope to merge any relevant changes back in.
+
+## usage
+
+The `compiler` directory contains the C language plugin (`capnpc-c`) for use with the `capnp` tool: https://capnproto.org/capnp-tool.html.
+
+`capnp` will by default search `$PATH` for `capnpc-c` - if it's on your PATH, you can generate code for your schema as follows:
+
+    $ capnp compile -o c myschema.capnp
+
+Otherwise, you can specify the path to the c plugin:
+
+    $ capnp compile -o ./capnpc-c myschema.capnp
+
+`capnp` generates a C struct that corresponds to each capn proto struct, along with read/write functions that convert to/from capn proto form.
+
+TBD whether it will make sense in the future to provide accessor functions for struct members, rather than converting entire structs.