DOCS: Added the missing provider OpenSRS page (#2665)

documentation/providers/opensrs.md

@@ -0,0 +1,19 @@
+## Configuration
+
+To use this provider, add an entry to `creds.json` with `TYPE` set to `OpenSRS`
+along with your OpenSRS credentials.
+
+## Usage
+
+An example configuration:
+
+{% code title="dnsconfig.js" %}
+```javascript
+var REG_NONE = NewRegistrar("none");
+var DSP_OPENSRS = NewDnsProvider("opensrs");
+
+D("example.com", REG_NONE, DnsProvider(DSP_OPENSRS),
+    A("test", "1.2.3.4")
+);
+```
+{% endcode %}