Update "Install OS X prerequisites using Homebrew" section of `prepare.md`

When I install gdk as written in "the Option 2: Develop in the main repo" (article says run gdk install), it tells me to only install pkg-config (brew install pkg-config). But in fact, I also need to be linking pkg-config (brew link pkg-config).

If I didn't link pkg-config which I had installed, my environment showed the following message.

current directory: <home directory>/.rvm/gems/ruby-2.5.3/gems/rugged-0.27.5/ext/rugged
<home directory>/.rvm/rubies/ruby-2.5.3/bin/ruby -r ./siteconf20190212-4251-y5cs9g.rb extconf.rb
.......
checking for pkg-config... no
.......

So in order to recognize pkg-config has been installed and linked, I think, we need to add the execution of brew link pkg-config in "Install OS X prerequisites using Homebrew" section.