Add SECURITY.md

Author: klassenjs

SECURITY.md

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+# GeoMoose Security Policy
+
+## Reporting a Vulnerability in GeoMoose
+
+Security/vulnerability reports should not be submitted through GitHub tickets or the public mailing lists, but instead please send your report 
+to the email address: **geomoose-security nospam @ osgeo.org** (remove the blanks and ‘nospam’).  
+
+## Supported Versions
+
+Only the current release of GeoMoose is supported (see https://www.geomoose.org/download.html).
+
+## Version Numbering: Explained
+
+version x.y.z means:
+
+**x**
+- Major release series number.
+- Major releases indicate substantial changes to the software and 
+  backwards compatibility is not guaranteed across series. Current 
+  release series is 3.
+
+**y**
+- Minor release series number.
+- Minor releases indicate smaller, functional additions or improvements 
+  to the software and should be generally backwards compatible within a 
+  major release series. Users should be able to confidently upgrade 
+  from one minor release to another within the same release series, so 
+  from 3.13.x to 3.14.x.
+
+**z**
+- Point release series number.
+- Point releases indicate maintenance releases - usually a combination of 
+  bug and security fixes and perhaps small feature additions. Backwards 
+  compatibility should be preserved and users should be able to confidently 
+  upgrade between point releases within the same release series, 
+  so from 3.14.0 to 3.14.15.