README.md: add build details

README.md

@@ -1,3 +1,28 @@
-# Tailscale cross-language embedding
+# libtailscale
 
-This package contains C bindings for tsnet, an in-process Tailscale node.
+libtailscale is a C library that embeds Tailscale into a process.
+
+Use this library to compile Tailscale into your program and get
+an IP address on a tailnet, entirely from userspace.
+
+## Building
+
+With the latest version of Go, run:
+
+```
+go build -buildmode=c-archive
+```
+
+This will produce a `libtailscale.a` file. Link it into your binary,
+and use the `tailscale.h` header to reference it.
+
+It is also possible to build a shared library using
+
+```
+go build -buildmode=c-shared
+```
+
+## Bugs
+
+Please file any issues about this code or the hosted service on
+[the issue tracker](https://github.com/tailscale/tailscale/issues).