Add addition documentation for hosts plugin, fix #1825 (#1836)

Signed-off-by: Mario Kleinsasser <mario.kleinsasser@gmail.com>

Rework the documentation
Signed-off-by: Mario Kleinsasser <mario.kleinsasser@gmail.com>

plugin/hosts/README.md

@@ -6,13 +6,33 @@
 
 ## Description
 
-The hosts plugin is useful for serving zones from a /etc/hosts file. It serves from a preloaded
+The hosts plugin is useful for serving zones from a `/etc/hosts` file. It serves from a preloaded
 file that exists on disk. It checks the file for changes and updates the zones accordingly. This
 plugin only supports A, AAAA, and PTR records. The hosts plugin can be used with readily
 available hosts files that block access to advertising servers.
 
 This plugin can only be used once per Server Block.
 
+## The hosts file
+
+Commonly the entries are of the from `IP_address canonical_hostname [aliases...]` as explained by the hosts(5) man page.
+
+Examples:
+
+~~~
+# The following lines are desirable for IPv4 capable hosts
+127.0.0.1       localhost
+192.168.1.10    example.com            example
+
+# The following lines are desirable for IPv6 capable hosts
+::1                     localhost ip6-localhost ip6-loopback
+fdfc:a744:27b5:3b0e::1  example.com example
+~~~
+
+### PTR records
+
+PTR records for reverse lookups are generated automatically by CoreDNS (based on the hosts file entries) and cannot be created manually. 
+
 ## Syntax
 
 ~~~
@@ -74,3 +94,7 @@ Load hosts file inlined in Corefile.
     }
 }
 ~~~
+
+## See also
+
+The form of the entries in the `/etc/hosts` file are based on IETF [RFC 952](https://tools.ietf.org/html/rfc952) which was updated by IETF [RFC 1123](https://tools.ietf.org/html/rfc1123).