Add HowTo whitelist signature / get info about signatur.

DOC/clamav/README.get-signature-info

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+# ----------
+# Get signature info (decode signatur)
+# ----------
+
+# - See also: https://lists.gt.net/clamav/users/73547
+
+# - If yoe see something like
+# -
+# -    May 15 15:49:56 mx amavis[19815]: (19815-17) Blocked INFECTED (MBL_27966083.UNOFFICIAL)
+# -
+# - you can get some infos about the blockin signatur (MBL_27966083.UNOFFICIAL)
+# - using clamav's 'sigtool' (ommit '.UNOFFICIAL' if present:
+# -
+sigtool --find-sigs MBL_27966083 |  sigtool --decode-sigs
+sigtool --find-sigs=MBL_27966083 |  sigtool --decode-sigs
+
+# - Output of that command is:
+# -
+VIRUS NAME: MBL_27966083
+DECODED SIGNATURE:
+https://docs.google.com
+