Create SECURITY.md for security policy

[t5xpnmfpyb-lgtm]

Added a security policy document outlining supported versions and vulnerability reporting.

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[eth-bot]

File SECURITY.md

Requires 2 more reviewers from @g11tech, @lightclient, @SamWilsn, @xinbenlv

[g11tech]

problem with having this is that when an EIP goes final its no more editable and this section can then become a nightmare with obsolete data

[g11tech]

cc @SamWilsn @poojaranjan