Added security policy

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Currently there are two supported branches:
+- 2.1.x for php 7.1 and above
+- 2.0.x for php 7.0 and below
+
+Please be sure to keep your PHP installation up-to-date with recommended versions.
+
+| Version | Supported          | PHP version | Support Ends |
+| ------- | ------------------ | ----------- | ------------ |
+| 2.2.x   | :x:                | >=7.2       | (WIP)        |
+| 2.1.x   | :white_check_mark: | >=7.1       | 01 Jun 2020  |
+| 2.0.x   | :white_check_mark: | <=7.0       | 01 Jun 2020  |
+| < 2.0   | :x:                | <=5.6       | 01 Jun 2018  |
+
+## Reporting a Vulnerability
+
+Please report found vulnerabilities directly to my email: [email protected]  
+You can use my public key [0xBF7537E8](https://pgp.denpa.pro/) (Fingerprint=B371 A23C C449 703F F1BF 0927 9CF6 463C BF75 37E8) to send me an encrypted message.