new(contrib): add elftoolchain fork readme

Signed-off-by: Luca Guerra <[email protected]>

contrib/README.md

@@ -0,0 +1,25 @@
+# Contrib directory
+
+This directory contains forks of some projects that may not be included as-is and require custom patches.
+
+## Elftoolchain
+
+This is the version of Elftoolchain (https://sourceforge.net/projects/elftoolchain/) that is used to satisfy the `libelf` requirement of the Falco libraries and its dependent `libbpf`.
+
+This is how this fork was created:
+
+* We downloaded the code from https://sourceforge.net/projects/elftoolchain/
+* We precompiled the autogenerated files by running:
+
+```sh
+cd common/sys
+m4 -I$(pwd) -D SRCDIR=$(pwd)   elfdefinitions.m4 > elfdefinitions.h
+cd ../../libelf
+m4 -D SRCDIR=$(pwd)  libelf_fsize.m4 > libelf_fsize.c
+m4 -D SRCDIR=$(pwd)  libelf_msize.m4 > libelf_msize.c
+m4 -D SRCDIR=$(pwd)  libelf_convert.m4 > libelf_convert.c
+```
+
+* We wrote `CMakeLists.txt` and `libelf/CMakeLists.txt` to enable CMake to build `libelf`
+* We added compatibility patches `elf.h`, `gelf_getnote.c`, `gelf_versym.c` and modified `gelf.h`, `libelf.h` to make this flavor of libelf work with libbpf
+* This way, a new target `elf` is built and can be linked via `target_link_libraries()`