Add initial README.md

Provide some basic documentation.
Signed-off-by: Shawn Webb <shawn.webb@hardenedbsd.org>

README.md

+# libpushover
+
+libpushover is a modern C library for the
+[Pushover](https://pushover.net/) service. Since its primary use case
+is with HardenedBSD's infrastructure monitoring daemon, portability to
+other operating systems (eg, Linux) is not a priority. However, if
+someone wants to punish themselves by porting to Linux, patches are
+gladly accepted for review.
+
+libpushover depends on only one library: libcurl. Eventual migration
+away from libcurl to libfetch is planned.
+
+## Example Usage
+
+The following code sample illustrates how easy it is to use
+libpushover:
+
+``
+#include <err.h>
+#include <libpushover.h>
+
+pushover_message_t *msg;
+pushover_ctx_t *ctx;
+
+ctx = pushover_init_ctx("Pushover API Token");
+if (ctx == NULL) {
+	errx(1, "Pushover init failed.");
+}
+
+msg = pushover_init_message(NULL);
+if (pmsg == NULL) {
+	errx("Pushover message init failed.");
+}
+
+pushover_message_set_dest("Destination token");
+pushover_message_set_title("Message title");
+pushover_message_set_msg("Message body content");
+pushover_submit_message(ctx, msg);
+
+pushover_free_message(&msg);
+pushover_free_ctx(&ctx);
+``