Added CAA_BUILDER (#478)

* Added CAA_BUILDER

* CAA-Builder: simply use "none" to refuse all CAs

* Fixed small error in documentation

docs/caa-builder.md

@@ -0,0 +1,46 @@
+---
+layout: default
+title: CAA Builder
+---
+
+# CAA Builder
+
+dnscontrol contains a CAA_BUILDER which can be used to simply create
+CAA records for your domains. Instead of creating each CAA record
+individually, you can simply configure your report mail address, the
+authorized certificate authorities and the builder cares about the rest.
+
+
+## Example
+
+For example you can use:
+
+```
+CAA_BUILDER({
+  label: "@",
+  iodef: "test@domain.tld",
+  iodef_critical: true,
+  issue: [
+    "letsencrypt.org",
+    "comodoca.com",
+  ],
+  issuewild: "none",
+})
+```
+
+The parameters are:
+
+* `label:` The label of the CAA record. (Optional. Default: `"@"`)
+* `iodef:` Report all violation to configured mail address.
+* `iodef_critical:` This can be `true` or `false`. If enabled and CA does not support this record, then certificate issue will be refused. (Optional. Default: `false`)
+* `issue:` An array of CAs which are allowed to issue certificates. (Use `"none"` to refuse all CAs)
+* `issuewild:` An array of CAs which are allowed to issue wildcard certificates. (Can be simply `"none"` to refuse issuing wildcard certificates for all CAs)
+
+`CAA_BUILDER()` returns multiple records (when configured as example above):
+
+  * `CAA("@", "iodef", "test@domain.tld", CAA_CRITICAL)`
+  * `CAA("@", "issue", "letsencrypt.org")`
+  * `CAA("@", "issue", "comodoca.com")`
+  * `CAA("@", "issuewild", ";")`
+
+```