Create a guide explaining how to use MullvadVPN with dnsx! (#734)

* A guide explaining how to use dnsx with Mullvad

I had the same problem using dnsx as in #221 and I successfully found a way around Mullvad's limitations. Wanted to share my solution.

* Add the solution to the README.md

MULLVAD.md

@@ -0,0 +1,8 @@
+# Usage with [MullvadVPN](https://mullvad.net/)
+As explained in [#221](https://github.com/projectdiscovery/dnsx/issues/221), VPN operators sometimes filter high DNS/UDP traffic. To avoid packet loss you can tweak a few settings in the client app.
+
+- 1. Go to *Settings > VPN settings* and set Wireguard as the default tunnel (the settings we need are only available with Wireguard).
+- 2. Still in the same section, raise the MTU (=maximum transmission; determines the largest packet size that can be transmitted through your network) to its maximum of 1420.
+- 3. Go back to *Settings > VPN settings* and add a custom DNS server (e.g Cloudfare's 1.1.1.1 & 1.0.0.1). It will disable 'DNS Protection' which is not a problem: it won't mess with our traffic system.
+
+Happy hacking! s/o to Saber for letting me know about [#221](https://github.com/projectdiscovery/dnsx/issues/221), helped me solve this problem & understand it. If this doesn't solve the problem, open an issue and tag me (@noctisatrae).

README.md

@@ -464,6 +464,6 @@ func main() {
 - Domain name (`wd`) input is mandatory for wildcard elimination.
 - DNS record flag can not be used when using wildcard filtering.
 - DNS resolution (`l`) and DNS brute-forcing (`w`) can't be used together.
-- VPN operators tend to filter high DNS/UDP traffic, therefore the tool might experience packets loss (eg. [Mullvad VPN](https://github.com/projectdiscovery/dnsx/issues/221))
+- VPN operators tend to filter high DNS/UDP traffic, therefore the tool might experience packets loss (eg. [Mullvad VPN](https://github.com/projectdiscovery/dnsx/issues/221)). Check [this potential solution](./MULLVAD.md).
 
 `dnsx` is made with 🖤 by the [projectdiscovery](https://projectdiscovery.io) team.