README: add BTF and CO-RE info

Add list of Linux distributions with kernel BTF built-in.
Give few useful links to BPF CO-RE-related material to help users get started.

README.md

@@ -52,7 +52,7 @@ $ PKG_CONFIG_PATH=/build/root/lib64/pkgconfig DESTDIR=/build/root make install
 ```
 
 Distributions
-=====
+=============
 
 Distributions packaging libbpf from this mirror:
   - [Fedora](https://src.fedoraproject.org/rpms/libbpf)
@@ -74,8 +74,51 @@ Package dependencies of libbpf, package names may vary across distros:
   - zlib
   - libelf
 
+BPF CO-RE (Compile Once – Run Everywhere)
+=========================================
+
+Libbpf supports building BPF CO-RE-enabled applications, which, in contrast to
+[BCC](https://github.com/iovisor/bcc/), do not require Clang/LLVM runtime
+being deployed to target servers and doesn't rely on kernel-devel headers
+being available.
+
+It does rely on kernel to be built with [BTF type
+information](https://www.kernel.org/doc/html/latest/bpf/btf.html), though.
+Some major Linux distributions come with kernel BTF already built in:
+  - Fedora 31+
+  - RHEL 8.2+
+  
+If your kernel doesn't come with BTF built-in, you'll need to build custom
+kernel. You'll need:
+  - `pahole` 1.16+ tool (part of `dwarves` package), which performs DWARF to
+    BTF conversion;
+  - kernel built with `CONFIG_DEBUG_INFO_BTF=y` option;
+  - you can check if your kernel has BTF built-in by looking for
+    `/sys/kernel/btf/vmlinux` file:
+  
+```shell
+$ ls -la /sys/kernel/btf/vmlinux
+-r--r--r--. 1 root root 3541561 Jun  2 18:16 /sys/kernel/btf/vmlinux
+```
+  
+To develop and build BPF programs, you'll need Clang/LLVM 10+. The following
+distributions have Clang/LLVM 10+ installed by default:
+  - Fedora 32+
+  - Ubuntu 20.04+
+  
+Otherwise, please make sure to update it on your system.
+
+The following resources are useful to understand what BPF CO-RE is and how to
+use it:
+- [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)
+- [libbpf-tools in BCC repo](https://github.com/iovisor/bcc/tree/master/libbpf-tools)
+  contain lots of real-world tools converted from BCC to BPF CO-RE. Consider
+  converting some more to both contribute to the BPF community and gain some
+  more experience with it.
+
 License
-=====
+=======
 
 This work is dual-licensed under BSD 2-clause license and GNU LGPL v2.1 license.
 You can choose between one of them if you use this work.