Add doc for external validation

docs/eprik-config.md

@@ -80,4 +80,13 @@ for EPRIK instead of CARA INT for a PMP integration for the following XDS transa
 | CMPD [[PHARM-1]](https://www.ihe.net/uploadedFiles/Documents/Pharmacy/IHE_Pharmacy_Suppl_CMPD.pdf) | /eprik-cara/camel/pmp-int/pmp/services/cmpd/chpharm1  |
 
 The other PMP host is available at the path `/eprik-cara/camel/pmp-dev/pmp/...`; see 
-https://cara-ch.github.io/emed-service-guide/endpoints/ for more details.
+https://cara-ch.github.io/emed-service-guide/endpoints/ for more details.
+
+## External validation
+
+The EPRIK can use the Swiss EVS Client validator to validate various transactions and resources (like XUA assertions 
+and ATNA messages). Due to limited resources and rate limiting, the external validation is disabled by default. You 
+can enable it for a specific client in the _Clients_ table. It will stay enabled for 8 hours and will be disabled 
+automatically.
+
+If the external validation is not enabled, a warning will be shown in the _Validation_ part of the transaction details.