diff --git a/packet-api/README.md b/packet-api/README.md
index c29ab38..771b366 100644
--- a/packet-api/README.md
+++ b/packet-api/README.md
@@ -16,4 +16,6 @@ export RUBYLIB=$GUIX_ENVIRONMENT/lib:$GUIX_ENVIRONMENT/lib/ruby/site_ruby
 export SSL_CERT_DIR=$GUIX_PROFILE/etc/ssl/certs
 export SSL_CERT_FILE=$GUIX_PROFILE/etc/ssl/certs/ca-certificates.crt
 export GIT_SSL_CAINFO="$SSL_CERT_FILE"
+# this helps when building things that depend on libxml2
+C_INCLUDE_PATH=$C_INCLUDE_PATH:$C_INCLUDE_PATH/libxml2
 ```