DOC: lua-api: Add documentation about lua filters

Lua filter support is highly experimental. The documentation was added to
allow first lua filter implementations. The API is not stabilized and must
be improved to be fully usable. This docuementation is quite light for
now. But more will be added.
diff --git a/.gitignore b/.gitignore
index ec9dd62..ef880c3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -52,3 +52,4 @@
 dev/hpack/gen-rht
 /src/dlmalloc.c
 /tests/test_hashes
+doc/lua-api/_build