Point to Element security@ address and policy

[davidegirardi]

Type of change

• Feature
• Bugfix
• Technical
• Other: update the security policy for the repo

Content

Security reporting email address and link to the Security Disclosure Policy.

Motivation and context

The current links point to the Matrix foundation.

Screenshots / GIFs

Checklist

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/element-hq/element-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

[bmarty]

LGTM, thanks!