Fill out the grains docs a bit

doc/ref/grains.rst

@@ -2,4 +2,37 @@
 Grains
 ======
 
-Static information that Salt derives about the system it's running on.
+Salt comes with an interface to derive information about the underlying system.
+This is called the grains interface, because it presents salt with grains of
+information.
+
+The grains interface is made available to Salt modules and components so that
+the right salt minion commands are automatically available on the right
+systems.
+
+It is important to remember that grains are bits of information loaded when
+the salt minion starts, so this information is static. This means that the
+information in grains is unchanging, therefore the nature of the data is
+static. So grains information are things like the running kernel, or the
+operating system.
+
+Writing Grains
+==============
+
+Grains are easy to write, the grains interface is derived my executing all of
+the "public" functions found in the modules located in the grains package.
+The functions in the modules of the grains must return a python dict, the keys
+in the dict are the names of the grains, the values are the values.
+
+This means that the actual grains interface is simply a python dict.
+
+Before adding a grain to salt, consider what the grain is and remember that 
+grains need to be static data.
+
+Examples of Grains
+------------------
+
+The core module in the grains package is where the main grains are loaded by
+the salt minion and the principal example of how to write grains:
+
+https://github.com/thatch45/salt/blob/master/salt/grains/core.py