Add MacPorts and pkgsrc package manager options (#524)

There is no standard package manager for MacOS, there are at least
three popular ones, and all of them have Rust available.  This pull
requests adds some of the other popular package managers.

Author: jornane

src/infra/other-installation-methods.md

@@ -21,8 +21,9 @@ might one _not_ want to install using those instructions?
   demand. If you need to install Rust without access to the internet, `rustup`
   is not suitable.
 - Preference for the system package manager. On Linux in particular, but also on
-  macOS with [Homebrew], and Windows with [Chocolatey] or [Scoop], developers sometimes
-  prefer to install Rust with their platform's package manager.
+  macOS with [Homebrew], [MacPorts] or [pkgsrc], and Windows with [Chocolatey]
+  or [Scoop], developers sometimes prefer to install Rust with their platform's
+  package manager.
 - Preference against `curl | sh`. On Unix, we usually install `rustup` by
   running a shell script via `curl`. Some have concerns about the security of
   this arrangement and would prefer to download and run the installer
@@ -111,6 +112,8 @@ Past releases can be found in [the archives].
 [`rustup-init.exe`]: https://static.rust-lang.org/rustup/dist/i686-pc-windows-gnu/rustup-init.exe
 [`rustup-init.sh`]: https://static.rust-lang.org/rustup/rustup-init.sh
 [homebrew]: http://brew.sh/
+[macports]: https://www.macports.org/
+[pkgsrc]: https://pkgsrc.joyent.com/install-on-osx/
 [chocolatey]: http://chocolatey.org/
 [scoop]: https://scoop.sh/
 [three tiers]: https://doc.rust-lang.org/nightly/rustc/platform-support.html