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 #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.

readme.md

@@ -403,6 +403,8 @@ export default () =>
 
 If child is an `<a>` tag and doesn't have a href attribute we specify it so that the repetition is not needed by the user. However, sometimes, you’ll want to pass an `<a>` tag inside of a wrapper and the `Link` won’t recognize it as a *hyperlink*, and, consequently, won’t transfer its `href` to the child. In cases like that, you should define a boolean `passHref` property to the `Link`, forcing it to expose its `href` property to the child.
 
+**Please note**: using a tag other than `a` and failing to pass `passHref` may result in links that appear to navigate correctly, but, when being crawled by search engines, will not be recognized as links (owing to the lack of `href` attribute). This may result in negative effects on your sites SEO.
+
 ```jsx
 import Link from 'next/link'
 import Unexpected_A from 'third-library'
@@ -730,6 +732,21 @@ Supported options:
 
 Then, change your `start` script to `NODE_ENV=production node server.js`.
 
+#### Disabling file-system routing
+By default, `Next` will serve eacy file in `/pages` under a pathname matching the filename (eg, `/pages/some-file.js` is served at `site.com/some-file`.
+
+If your project uses custom routing, this behavior may result in the same content being served from multiple paths, which can present problems with SEO and UX. 
+
+To disable this behavior & prevent routing based on files in `/pages`, simply set the following option in your `next.config.js`: 
+
+```js
+// next.config.js
+module.exports = {
+  useFileSystemPublicRoutes: false
+}
+```
+
+
 ### Dynamic Import
 
 <p><details>