Merge pull request #1839 from justinmayer/persistent-configuration

Add docs on persisting settings across upgrades

docs/Formula-Cookbook.md

@@ -783,6 +783,8 @@ The symlinks created by `install_symlink` are guaranteed to be relative. `ln_s`
 
 For example, Ruby 1.9’s gems should be installed to `var/lib/ruby/` so that gems don’t need to be reinstalled when upgrading Ruby. You can usually do this with symlink trickery, or *better* a configure option.
 
+Another example would be configuration files that should not be overwritten on package upgrades. If after installation you find that to-be-persisted configuration files are not copied but instead *symlinked* into `/usr/local/etc/` from the Cellar, this can often be rectified by passing an appropriate argument to the package’s configure script. That argument will vary depending on a given package’s configure script and/or Makefile, but one example might be: `--sysconfdir=#{etc}`
+
 ### launchd plist files
 
 Homebrew provides two Formula methods for launchd plist files. [`plist_name`](http://www.rubydoc.info/github/Homebrew/brew/master/Formula#plist_name-instance_method) will return e.g. `homebrew.mxcl.<formula>` and [`plist_path`](http://www.rubydoc.info/github/Homebrew/brew/master/Formula#plist_path-instance_method) will return e.g. `/usr/local/Cellar/foo/0.1/homebrew.mxcl.foo.plist`.