Add an option to target different files

[fbuireu]

By default (as it should be) it uploads automatically the README.md file. I imagine some other scenarios where someone may want to update another file (let's say README_it.md beside the activity will be displayed in English).

Currently, this is not an option because there's no option to choose the target file. I'd like the option to use different markdown files to be uploaded using a key like: TARGET_FILE.

I'll try to make a PR if my free time allows me.

Thanks for all the work!

[Lissy93]

I've submitted a PR for this, in #79 :)

[tuunit]

will me merge with #97