README: add a Fossies codespell badge

I keep forgetting where the report is. Hopefully the badge will
make it easier to find it. I also fixed several typos codespell
found along the way.

src/core/main.c

@@ -517,7 +517,7 @@ static int parse_proc_cmdline_item(const char *key, const char *value, void *dat
         } else if (!value) {
                 const char *target;
 
-                /* Compatible with SysV, but supported independently even if SysV compatiblity is disabled. */
+                /* Compatible with SysV, but supported independently even if SysV compatibility is disabled. */
                 target = runlevel_to_target(key);
                 if (target)
                         return free_and_strdup_warn(&arg_default_unit, target);