Doc security tag is disabled and fails if enabled

Document that the security tag is disabled by default. Also document
that there are currently no checks in the security tag, so enabling it
will fail.

$ ./gocritic check -v -enable='#security
init checkers: empty checkers set selected
$ echo $?
1

When trying to enable the tag through golangci-lint, this is the error:

ERRO Invalid gocritic settings: gocritic [enabled]tag "security" doesn't
exist

The tag was added in #989.

Signed-off-by: Daniel Farrell <dfarrell@redhat.com>

README.md

@@ -109,7 +109,7 @@ Tags:
 * `#performance` - kind of checks that detect potential performance issues in code
 * `#experimental` - check is under testing and development. Disabled by default
 * `#opinionated` - check can be unwanted for some people. Disabled by default
-* `#security` -  kind of checks that find security issues in code
+* `#security` -  kind of checks that find security issues in code. Disabled by default and empty, so will fail if enabled.
 
 ## Contributing