Document parameters

[rmartinho]

The documentation needs to be updated for #56 and #63.

[rmartinho]

Btw, the docs at https://nonius.io are always built from the stable branch, but the current devel docs can be found at https://nonius.io/devel (they are rebuilt daily if there are changes).

[mastensg]

for anyone coming here because they can't find documentation on parameters elsewhere (like me). for https://github.com/libnonius/nonius/blob/devel/examples/example6.c%2B%2B:

./example6 -p size:200 -p string:bar

i discovered this by guessing at a simpler version of the syntax described in #56 (comment)