give a warning in documentation if syslog-logs parser is missing (#120)

docs/write_configurations/parsers.md

@@ -3,6 +3,12 @@
 !!! info
     Please ensure that you have working env or setup test environment before writing your parser.
 
+!!! warning "Parser dependency"
+
+The crowdsecurity/syslog-logs parsers is needed by the core parsing
+engine. Deletion or modification of this could result of {{crowdsec.name}}
+being unable to parse logs, so this should be done very carefully.
+
 > In the current example, we'll write a parser for the logs produced by `iptables` (netfilter) with the `-j LOG` target.
 > This document aims at detailing the process of writing and testing new parsers.
 
@@ -410,4 +416,4 @@ statics:
   - meta: http_path
     expression: "evt.Parsed.request"
 ```
-</details> -->
+</details> -->