Update the GitHub issue template

Set more emphasis on security reports.

[skip ci]

Author: fxlb

.github/ISSUE_TEMPLATE

@@ -1,4 +1,4 @@
-To report a security issue please send an e-mail to security@tcpdump.org.
+To report a ***S_E_C_U_R_I_T_Y issue please send an e-mail*** to security@tcpdump.org.
 
 To report bugs and other problems, contribute patches, request a
 feature, provide generic feedback etc
@@ -10,3 +10,5 @@ with 'tcpdump --version' or './tcpdump --version'
 
 If needed, you can attach to a comment a POC pcap file as a gzip compressed
 file or in a zip archive.
+
+- [ ] This is not a security issue (See first line).