Pull request #60: document the $dnstype modifier

Merge in DNS/adguard-home-wiki from 2337-dnstype to master

Closes #2337.

Squashed commit of the following:

commit 36f22ef8f9374d956337f99b75a7e5da882df072
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Nov 24 21:17:13 2020 +0300

    document the $dnstype modifier

Hosts-Blocklists.md

@@ -7,6 +7,7 @@
     * [Regular expressions support](#regular-expressions)
     * [Rule modifiers](#modifiers)
         * [client](#client)
+        * [dnstype](#dnstype)
         * [important](#important)
         * [badfilter](#badfilter)
         * [ctag](#ctag)
@@ -98,7 +99,7 @@ Example:
 
 > **IMPORTANT:** If a rule contains a modifier not listed in this document, the whole rule **must be ignored**. This way we will avoid false-positives when people are trying to use unmodified browser ad blockers' filter lists like EasyList or EasyPrivacy.
 
-#### <a id="client"></a> <a id="client"></a> `client`
+#### <a id="client"></a> `client`
 
 The `$client` modifier allows specifying clients this rule will be working for. It accepts both client names, IP addresses or CIDR ranges.
 
@@ -128,6 +129,39 @@ Client names usually contain spaces or other special characters, that's why you
 * `||example.org^$client=~Mom|~Dad|Kids` -- block `example.org` for `Kids`, but not for `Mom` and `Dad`. This example demonstrates how to specify multiple clients in one rule.
 * `||example.org^$client=192.168.0.0/24` -- block `example.org` for all clients with IP addresses in the range `192.168.0.0-192.168.0.255`
 
+#### <a id="dnstype"></a> `dnstype`
+
+The `$dnstype` modifier allows specifying DNS request type on which this rule
+will be triggered.
+
+The syntax is:
+
+```
+$dnstype=value1|value2|...
+$dnstype=~value1|~value2|~...
+```
+
+The names of the types are case-insensitive, but are validated against a set of
+actual DNS resource record (RR) types.
+
+This:
+
+```
+$dnstype=~value1|value2
+```
+
+Is equivalent to this:
+
+```
+$dnstype=value2
+```
+
+**Examples**
+
+* `||example.org^$dnstype=AAAA` — Block IPv6 DNS requests for `example.org`.
+* `||example.org^$dnstype=~A|~CNAME` — Only allow `A` and `CNAME` DNS requests
+  for `example.org`, block out the rest.
+
 #### <a id="important"></a> <a id="important"></a> `important`
 
 The `$important` modifier applied to a rule increases its priority over any other rule without \$important modifier. Even over basic exception rules.
@@ -264,4 +298,4 @@ What it's capable of:
 
 1. Compile a single blocklist from multiple sources.
 2. Exclude the rules you don't need.
-3. Cleanup the resulting list: deduplicate, remove invalid rules, compress the list.
+3. Cleanup the resulting list: deduplicate, remove invalid rules, compress the list.