add content-length warning to FAQ

doc/manual/FAQ.markdown

@@ -272,4 +272,19 @@ perhaps because it's not available to us. In the meantime you can
 approximate a has_one by using a has_many with validates_length_of to
 constrain it to a single item.
 
-That being said, Peter Pavlovich is working on support for has_one.
+That being said, Peter Pavlovich is working on support for has_one.
+
+# How do I get rid of `WARN  Could not determine content-length of response body. Set content-length of the response or set Response#chunked = true`
+
+This one seems to be an issue with WEBrick and Ruby 1.9.3.
+
+I got rid of it by converting to using
+[Thin](http://code.macournoyer.com/thin/) instead of WEBrick. Seems
+like a snappier webserver anyway.
+
+BTW: Since I am on Windows installing Thin may be difficult, due to
+its dependency on eventmachine. However, I followed [this
+tip](http://stackoverflow.com/questions/3649252/cannot-install-thin-on-windows#comment10860757_4200880)
+on a stackoverflow answer, which worked for me (I used version
+1.0.0.rc.4). Future releases of eventmachine may have fixed this
+problem though.