Add initial cooke information to documentation (#25)

Adds initial information on `_panda_cms_session` cookie to
documentation.

docs/developers/cookies.md

@@ -0,0 +1,14 @@
+---
+title: Cookies
+layout: default
+parent: Privacy
+---
+
+## Cookies
+
+The following cookies are set through use of the system. We recommend, for GDPR compliance, you mark them as follows:
+
+### _panda_cms_session
+
+**Cookie Type:** Necessary
+**Description:** This cookie is necessary for website functionality. It allows ensuring that your user session is routed appropriately to load balancers and does not track any personally identifiable information. Functionality for this cookie may differ when you are logged in, at which point you have given your consent and it is a functional cookie.