Move common cr-policy-indicator behavior into CrPolicyIndicatorBehavior

ui/webui/resources/cr_elements/policy/README.md

Index: ui/webui/resources/cr_elements/policy/README.md
diff --git a/ui/webui/resources/cr_elements/policy/README.md b/ui/webui/resources/cr_elements/policy/README.md
new file mode 100644
index 0000000000000000000000000000000000000000..6f10d00bfaba2202440d1d127b9231d5081a33de
--- /dev/null
+++ b/ui/webui/resources/cr_elements/policy/README.md
@@ -0,0 +1,13 @@
+Policy indicators show when a setting is controlled by something other than the
+user, especially user/device policy. The badge icons are sourced from
+[cr_elements/icons.html](../icons.html) by default. Indicators show a tooltip on
+hover when `CrPolicyStrings` is defined.
+
+Elements like `<cr-policy-pref-indicator>` and `<cr-policy-network-indicator>`
+are designed to be reused in WebUI pages:
+
+    <cr-policy-pref-indicator pref="[[pref]]"></cr-policy-pref-indicator>
+
+For one-off or composed elements, CrPolicyIndicatorBehavior provides some
+configurable properties and calculates dependent properties, such as the
+tooltip, icon, and visibility of the indicator.

[dschuyler, 2017/02/28 20:09:29]

Just a suggestion, totally optional:
The above is a good overview of the features of this code, which is cool. To add
to it, I'd find it super useful to have a list of instructions and/or and
example to search for. e.g.

"To add a new indicator:
1. 
2."

or

"For an example of a correctly implemented indicator, search the code for
______."

[michaelpg, 2017/02/28 21:55:50]

Were you looking for instructions on creating a new indicator in policy/, or for
displaying a policy/ indicator elsewhere in the UI?

I've expanded the README a bit for clarity, LMK where it can be improved.