Add csaf_validator

docs/csaf_validator.md

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+## csaf_validator
+
+is a tool to validate local advisories files against the JSON Schema and an optional remote validator.
+
+### Usage
+
+```
+csaf_validator [OPTIONS] files...
+
+Application Options:
+      --version                Display version of the binary
+      --validator=URL          URL to validate documents remotely
+      --validatorcache=FILE    FILE to cache remote validations
+      --validatorpreset=       One or more presets to validate remotely (default: mandatory)
+
+Help Options:
+  -h, --help                   Show this help message
+```