ibuf: describe why ibuf_*() functions are copied

I wrote it mainly to clarify when the hack could be removed.

Follows up #59

third_party/README.md

@@ -0,0 +1,11 @@
+## memcached_ibuf
+
+It is the copy of ibuf from the small library with renaming of the functions.
+The motivatoin is to avoid possible name clash between tarantool's symbols and
+the module's symbols. See [1] and [2] for details.
+
+We can remove it, when we'll dedice to drop support of tarantool versions
+affected by [1].
+
+[1]: https://github.com/tarantool/tarantool/issues/6873
+[2]: https://github.com/tarantool/memcached/issues/59#issuecomment-1081106140