Add database API example

sample.c

@@ -0,0 +1,9 @@
+#include "database.h"
+
+int main() {
+    struct database_state state;
+    database_state_init(&state, "data.sqlite");
+    database_write_active(&state, 1, 1);
+    database_write_location(&state, 1, 5.0, 5.0);
+    database_state_free(&state);
+}