doc/fields-snmp: prepare a new extensive example

This should show the capabilities of filter-based fields based on a real-world
example: SNMP credentials based on chosen SNMP version

doc/16-Fields-example-SNMP.md

@@ -0,0 +1,51 @@
+Data Fields example: SNMP
+=========================
+
+Ever wondered how to provide an easy to use SNMP configuration to your users?
+That's what we're going to show in this example. Once completed, all your Hosts
+inheriting a specific (or your "default") Host Template will provide an optional
+`SNMP version` field.
+
+In case you choose no version, nothing special will happen. Otherwise, the host
+offers additional fields depending on the chosen version. `Community String` for
+`SNMPv1` and `SNMPv2c`, and five other fields ranging from `Authentication User`
+to `Auth` and `Priv` types and keys for `SNMPv3`.
+
+Your services should now be applied not only based on various Host properties
+like `Device Type`, `Application`, `Customer` or similar - but also based on
+the fact whether credentials have been given or not.
+
+Prepare required Data Fields
+----------------------------
+
+As we already have learned, `Fields` are what allows us to define which custom
+variables can or should be defined following which rules. We want SNMP version
+to be a drop-down, and that's why we first define a `Data List`, followed by
+a `Data Field` using that list:
+
+### Create a new Data List
+
+![Create a new Data List](screenshot/director/16_fields_snmp/161_snmp_versions_create_list.png)
+
+### Fill the new list with SNMP versions
+
+![Fill the new list with SNMP versions](screenshot/director/16_fields_snmp/162_snmp_versions_fill_list.png)
+
+### Create a corresponding Data Field
+
+![Create a Data Field for SNMP Versions](screenshot/director/16_fields_snmp/163_snmp_version_create_field.png)
+
+Next, please also create the following elements:
+
+* a list *SNMPv3 Auth Types* providing `MD5` and `AES`
+* a list *SNMPv3 Priv Types* providing at least `AES` and `DES`
+* a `String` type field `snmp_community` labelled *SNMP Community*
+* a `String` type field `snmpv3_user` labelled *SNMPv3 User*
+* a `String` type field `snmpv3_auth` labelled *SNMPv3 Auth* (authentication key)
+* a `String` type field `snmpv3_priv` labelled *SNMPv3 Priv* (encryption key)
+* a `Data List` type field `snmpv3_authprot` labelled *SNMPv3 Auth Type*
+* a `Data List` type field `snmpv3_privprot` labelled *SNMPv3 Priv Type*
+
+Please do not forget to add meaningful descriptions, telling your users about
+in-house best practices.
+