Added instructions for adding science tap for installing certain packages.

`brew install scipy` was failing with:

> Error: No available formula for suite-sparse 
Please tap it and then try again: brew tap homebrew/science
Searching taps...
Error: 404 Not Found
Please report this bug:
    https://github.com/mxcl/homebrew/wiki/troubleshooting
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/open-uri.rb:277:in `open_http'
/usr/local/Library/Homebrew/cmd/search.rb:99:in `value'
/usr/local/Library/Homebrew/cmd/search.rb:99:in `search_taps'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/open-uri.rb:230:in `inject'
/usr/local/Library/Homebrew/cmd/search.rb:96:in `each'
/usr/local/Library/Homebrew/cmd/search.rb:96:in `inject'
/usr/local/Library/Homebrew/cmd/search.rb:96:in `search_taps'
/usr/local/Library/Homebrew/cmd/install.rb:41:in `install'
/usr/local/Library/brew.rb:91:in `send'
/usr/local/Library/brew.rb:91

So just added extra instructions in readme.md