read secondary structure from file

[csbrasnett]

Rather than pasting a long string, it might be nice if there was an option to read the secondary structure from some kind of text file. This is slightly inspired by my PR and associated discussion for the website and tutorial, because I think the current explanation is confusing to people not familiar with martinize2.

Maybe this is more of a documentation problem. I started having a go at writing a wiki over the summer, but that still needs a bit more work. Thoughts welcome