docs: add remark to additional modifiers

[donkeyDau]

Information taken from #62

[amiller-gh]

Looks like the package doesn't support all of the remark modifier, just * ? and +:

preact-router/src/util.js

Line 30 in 5f00182

flags = (route[i].match(/[+*?]+$/) || EMPTY)[0] || '',

Would love to see full support though! Especially for unnamed path segments (ex: /foo(/bar)). Would help tremendously with matching default routes. @developit, interested in a PR?