NEW PROVIDER: Mythic Beasts DNS (#2528)

Co-authored-by: Tom Limoncelli <tlimoncelli@stackoverflow.com>

documentation/providers/mythicbeasts.md

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+This is the provider for [Mythic Beasts](https://www.mythic-beasts.com/) using its [Primary DNS API v2](https://www.mythic-beasts.com/support/api/dnsv2).
+
+## Configuration
+
+To use this provider, add an entry to `creds.json` with `TYPE` set to `MYTHICBEASTS` along with a Mythic Beasts API key ID and secret.
+
+Example:
+
+{% code title="creds.json" %}
+```json
+{
+  "mythicbeasts": {
+    "TYPE": "MYTHICBEASTS",
+	"keyID": "xxxxxxx",
+	"secret": "xxxxxx"
+  }
+}
+```
+{% endcode %}
+
+## Usage
+
+For each domain:
+
+* Domains must be added in the [web UI](https://www.mythic-beasts.com/customer/domains), and have DNS enabled.
+* In Mythic Beasts' DNS management web UI, new domains will have set a default DNS template of "Mythic Beasts nameservers only". You must set this to "None".
+
+An example configuration:
+
+{% code title="dnsconfig.js" %}
+```javascript
+var REG_NONE = NewRegistrar("none");
+var DSP_MYTHIC = NewDnsProvider("mythicbeasts");
+
+D("example.com", REG_NONE, DnsProvider(DSP_MYTHIC),
+    A("test", "1.2.3.4")
+);
+```
+{% endcode %}