Add Example in Readme for the tool/resampleres macro

[HG3112]

I'm looking for an automated cli tool to resize/crop/add margin to .ass subtitle files. Aegisub seem to do a good job via GUI.

However I'm not able to configure the resampleres box via CLI, which has been introduced in latest pre-release. From tkarabela/pysubs2#29 it seems that "Aegisub's resolution resampler is implemented in C++ and is not exposed in its Lua API", so I do not know where to find control name strings for --dialog config, as well as a lua file for --automation

For test/info I run the following command line (windows 10)

aegisub-cli --loglevel 4 input.ass out.ass tool/resampleres

and in the last line i get

> [main] Fatal error while initializing: No video loaded

which is probably an exception from resolution_resampler.cpp (line 234)

A command line example to start with could probably solve my problem.

Thanks
HG