Document plural "options" flag

[nkuehn]

What happened?

I needed to use multiple graalvm native-image options at once, but was stuck because the README only documents the singular "option", which (by looking into the implementation) adds to the plural "options".

What did you want to happen?

Plural "options" being documented with an example and not using plural in its description. I ended up using:

options.add("--allow-incomplete-classpath")
options.add("--report-unsupported-elements-at-runtime")
options.add("--no-fallback")

There may be a nicer version to achieve this