sys-process/audit: Add README.md

Try to list the reasons behind this for of the gentoo package.

sdk_container/src/third_party/coreos-overlay/sys-process/audit/README.md

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+This is a fork of gentoo's `sys-process/audit` package. The main
+reasons for having our fork seem to be:
+
+1. We have our own audit rules (see files in `files/rules.d`
+   directory).
+
+  - These seem to be mostly similar to what gentoo provides, but split
+    into several files and they have an additional rule for SELinux
+    events.
+
+  - We also install it in a different place and place symlinks with
+    systemd's tmpfiles functionality.
+
+2. We install a systemd service that loads our rules at startup.
+
+3. We add a `daemon` use flag that gates a build of `auditd` binary
+   and some more tools. This flag seems to be unused, which results in
+   the daemon and tools not being built. The role of auditd is to
+   write audit records to disk, and both ausearch and aureport utilize
+   those written logs. Since audit logs are also written to journal,
+   writing them to disk seems redundant, thus auditd and the tools
+   seem to be unnecessary. This also reduces the final image size a
+   bit.
+
+4. We don't do the permissions lockdown on some auditd files for some
+   reason. It's either related that we don't build auditd in practice
+   or it's about our own audit rules.