Add tip about getting the executable name from within an image

``` 
$ ./bb -e '(org.graalvm.nativeimage.ProcessProperties/getExecutableName)'
"/Users/borkdude/Dropbox/dev/clojure/babashka/bb"
```

See https://www.graalvm.org/sdk/javadoc/index.html?org/graalvm/nativeimage/ProcessProperties.html