libbpf-tools: add links to BPF CO-RE posts
Add links to BPF CO-RE blog posts, explainint what it is, how to use it, and
giving practical recommendations on how to convert BCC BPF program to
libbpf-based one.
+Useful links
+------------
+
+- [BPF Portability and CO-RE](https://facebookmicrosites.github.io/bpf/blog/2020/02/19/bpf-portability-and-co-re.html)
+- [HOWTO: BCC to libbpf conversion](https://facebookmicrosites.github.io/bpf/blog/2020/02/20/bcc-to-libbpf-howto-guide.html)
+
Building
-------
