doc: note on distro support for ctags with seccomp

Change-Id: Id6d8a2fdb6d59480ffb1ec38e123bce20e9c51d2

doc/ctags.md

diff --git a/doc/ctags.md b/doc/ctags.md
index ed1e6cf..36b4014 100644
--- a/doc/ctags.md
+++ b/doc/ctags.md
@@ -16,6 +16,11 @@
 which guarantees that security problems in ctags cannot escalate to
 access to the indexing machine.
 
+Ubuntu, Debian and Arch provide universal ctags with seccomp support
+compiled in. Zoekt expects the `universal-ctags` binary to be on
+`$PATH`. Note: only Ubuntu names the binary `universal-ctags`, while
+most distributions name it `ctags`.
+
 Use the following invocation to compile and install universal-ctags:
 
 ```