doc: externalize links

doc/README.md

@@ -1,18 +1,16 @@
 # wepoll - epoll for windows
 
-This library implements the
-[epoll](http://man7.org/linux/man-pages/man7/epoll.7.html) API for
-Windows applications. It attempts to be efficient, and to match the
-Linux API and as closely as possible.
+This library implements the [epoll][man epoll] API for Windows
+applications. It attempts to be efficient, and to match the Linux API
+as semantics as closely as possible.
 
 ## Rationale
 
 Unlike Linux, OS X, and many other operating systems, Windows doesn't
 have a good API for receiving socket state notifications. It only
 supports the `select` and `WSAPoll` APIs, but they
-[don't scale](https://daniel.haxx.se/docs/poll-vs-select.html)
-and suffer from
-[other issues](https://daniel.haxx.se/blog/2012/10/10/wsapoll-is-broken/).
+[don't scale][select scale] and suffer from
+[other issues][wsapoll broken].
 
 Using I/O completion ports isn't always practical when software is
 designed to be cross-platform. Wepoll offers an alternative that is
@@ -64,7 +62,7 @@ HANDLE epoll_create1(int flags);
 * `size` is ignored but most be greater than zero.
 * `flags` must be zero as there are no supported flags.
 * Returns `INVALID_HANDLE_VALUE` on failure.
-* [man page](http://man7.org/linux/man-pages/man2/epoll_create.2.html)
+* [man page][man epoll_create]
 
 ### epoll_close
 
@@ -94,7 +92,7 @@ int epoll_ctl(HANDLE ephnd,
   wepoll may not be able to detect this until the next call to
   `epoll_wait()`.
 * TODO: expand
-* [man page](http://man7.org/linux/man-pages/man2/epoll_ctl.2.html)
+* [man page][man epoll_ctl]
 
 ### epoll_wait
 
@@ -111,5 +109,11 @@ int epoll_wait(HANDLE ephnd,
   -  0 when a timeout occurs
   - ≥1 the number of evens received
 * TODO: expand
-* [man page](http://man7.org/linux/man-pages/man2/epoll_wait.2.html)
+* [man page][man epoll_wait]
 
+[man epoll]:        http://man7.org/linux/man-pages/man7/epoll.7.html
+[man epoll_create]: http://man7.org/linux/man-pages/man2/epoll_create.2.html
+[man epoll_ctl]:    http://man7.org/linux/man-pages/man2/epoll_ctl.2.html
+[man epoll_wait]:   http://man7.org/linux/man-pages/man2/epoll_wait.2.html
+[select scale]:     https://daniel.haxx.se/docs/poll-vs-select.html
+[wsapoll broken]:   https://daniel.haxx.se/blog/2012/10/10/wsapoll-is-broken/