add info about empty folder

external/ibgateway/readme.txt

@@ -0,0 +1,7 @@
+Put the IB Gateway installer file into this folder before you start the build.
+They are not part of the git repo to save space and bandwith.
+
+Donwload the x64 Linux version from the Interactive Brokers Homepage or 
+release history (https://github.com/waytrade/ib-gateway-docker/releases) and name it like this:
+
+ibgateway-<VERSION>-standalone-linux-x64.sh