doc'd fs-routing option & added note on `passHref` (#3384)
doc'd fs-routing option & added note on `passHref` (#3384)

2 changes:

`passHref` - just added a cautionary note on the importance of `passHref`. We had a few days of no-href links on our site b/c we used a custom component instead of a raw `<a>` tag,  and Google bot wasn't crawling our links (confirmed in Google cache). Hurt our SEO a bit, so I thought it was worth noting.


`useFileSystemPublicRoutes` - this is mentioned in https://github.com/zeit/next.js/pull/914 , but it doesn't appear any doc was actually added. We use `next-routes`, and we were serving all the files in `/pages/` in addition to their route patterns (ie duplicate content), which can be a pain w/ SEO and duplicate content.
JeromeFitz/next.js,BlancheXu/test,azukaru/next.js,JeromeFitz/next.js,BlancheXu/test,zeit/next.js,JeromeFitz/next.js,zeit/next.js,azukaru/next.js,flybayer/next.js,azukaru/next.js,azukaru/next.js,flybayer/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,zeit/next.js,BlancheXu/test