Add FAQ section

Author: zoff99

FAQ.md

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+# Overview
+
+Contains [frequently asked questions](https://en.wikipedia.org/wiki/FAQ)
+related to Trifa Tox Client.
+
+
+# List of Content
+
+1. [What are the circles next to a contact?](#what-are-the-circles-next-to-a-contact)
+
+
+# What are the circles next to a contact?
+
+There are 3 circles on the contact item, see
+
+![Example Trifa Contact List](./doc/trifa-contact-list.png)
+
+* the circle around avatar (`1`) tells the connection type, like
+    * <span style="background-color: green; color: white">green</span> -> friend is online via UDP
+    * <span style="background-color: yellow; color: black">yellow</span> -> friend is online via TCP
+    * <span style="background-color: gray; color: white">gray</span> -> friend is offline
+* the medium circle (`2`) tells the conncetion status, like
+    * <span style="background-color: green; color: white">green</span> -> friend has toxproxy (for offline messages) and proxy is online
+    * <span style="background-color: yellow; color: black">yellow</span> -> friend has PUSH notification feature
+    * <span style="background-color: red; color: white">red</span> -> friend has toxproxy (for offline messages) but it is offline
+* the smallest circle (`3`) tells the contact/user status, like
+    * <span style="background-color: green; color: white">green</span> -> friend set status to "I'm available"
+    * <span style="background-color: yellow; color: black">yellow</span> -> friend set status to "I'm away"
+    * <span style="background-color: red; color: white">red</span> -> friend set status to "I'm busy"
+
+
+