run-ci: document it

run-ci

 #!/usr/bin/env bash
 
+# head.hackage CI driver script
+# =============================
+#
+# Environment variables:
+#   GHC:           path to `ghc` executable (defaults to `ghc`)
+#   CI_CONFIG:     path to a configuration script (defaults to `config.sh`)
+#   EXTRA_OPTS:    extra options to pass to `head-hackage-ci`
+#   EXTRA_HC_OPTS: extra options to pass to `ghc`
+#   CPUS:          parallelism amount
+#   USE_NIX:       use `nix` to provide build dependencies.
+#   COLLECT_RTS_STATS
+#                  collect runtime system statistics from compiler (forced on)
+#   COLLECT_PERF_STATS
+#                  collect perf_events counters from compiler (forced on)
+#   GHC_PERF_STATS_EVENTS
+#                  which counters to collect
+
 set -e
 
 cd ci