Document package list config in readme.

readme.markdown

@@ -283,6 +283,10 @@ enhancements. For example:
 
 ```ini
 nimbleDir = r"C:\Nimble\"
+
+[PackageList]
+name = "CustomPackages"
+url = "http://mydomain.org/packages.json"
 ```
 
 You can currently configure the following in this file:
@@ -293,6 +297,12 @@ You can currently configure the following in this file:
   application packages. If ``true`` this will add ``chcp 65001`` to the
   .cmd stubs generated in ``~/.nimble/bin/``.
   **Default:** ``true``
+* ``[PackageList]`` + ``name`` + ``url`` - You can use this section to specify
+  a new custom package list. Multiple package lists can be specified. Nimble
+  defaults to the "Official" package list, you can override it by specifying
+  a ``[PackageList]`` section named "official". Multiple URLs can be specified
+  under each section, Nimble will try each in succession if
+  downloading from the first fails.
 
 ## Creating Packages