Create SECURITY.md for security policy and reporting

[gcours85-code]

Add a security policy document outlining supported versions and vulnerability reporting.{
"id": "x-mitre-collection--402e24b4-436e-4936-b19b-2038648f489",
"type": "x-mitre-collection",
"spec_version": "2.1",
"x_mitre_attack_spec_version": "2.1.0",
"name": "Enterprise ATT&CK",
"x_mitre_version": "6.2",
"description": "Version 6.2 of the Enterprise ATT&CK dataset",
"created_by_ref": "identity--c78cb6e5-0c4b-4611-8297-d1b8b55e40b5",
"created": "2018-10-17T00:14:20.652Z",
"modified": "2019-10-11T19:30:42.406Z",
"object_marking_refs": [
"marking-definition--fa42a846-8d90-4e51-bc29-71d5b4802168"
],
"x_mitre_contents": [
{
"object_ref": "attack-pattern--01a5a209-b94c-450b-b7f9-946497d91055",
"object_modified": "2019-07-17T20:04:40.297Z"
},
{
"object_ref": "attack-pattern--0259baeb-9f63-4c69-bf10-eb038c390688",
"object_modified": "2019-06-18T13:58:28.377Z"
},
{
"object_ref": "relationship--0024d82d-97ea-4dc5-81a1-8738862e1f3b",
"object_modified": "2019-04-24T23:59:16.298Z"
},
{
"object_ref": "intrusion-set--090242d7-73fc-4738-af68-20162f7a5aae",
"object_modified": "2019-03-22T14:21:19.419Z"
},
{
"object_ref": "malware--069af411-9b24-4e85-b26c-623d035bbe84",
"object_modified": "2019-04-22T22:40:40.953Z"
},
{
"object_ref": "identity--c78cb6e5-0c4b-4611-8297-d1b8b55e40b5",
"object_modified": "2017-06-01T00:00:00.000Z"
},
{
"object_ref": "marking-definition--fa42a846-8d90-4e51-bc29-71d5b4802168",
"object_modified": "2017-06-01T00:00:00.000Z"
}
]
}

[gcours85-code]

.
├─ enterprise-attack ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ [1] Collection folder for Enterprise
│ ├─ enterprise-attack.json ∙∙∙∙∙∙∙∙∙∙∙∙∙∙ [2] Most recent Enterprise release
│ ├─ enterprise-attack-9.0.json ∙∙∙∙∙∙∙∙∙∙ [3] Enterprise ATT&CK v9.0 collection
│ └─ [other releases of Enterprise ATT&CK]
├─ mobile-attack
│ └─ [Mobile ATT&CK releases]
├─ ics-attack
│ └─ [ATT&CK for ICS releases]
├─ index.json ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ [4] Collection index JSON
└─ index.md ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ [5] Collection index markdown