Added SHA1 & SHA256 application certificate fingerprints to README

[DanyPM]

Type of change

• Feature
• Bugfix
• Technical
• Other : README modification

Content

Add the app certificate fingerprint to the README file, enabling apps and users to check the integrity of apk installed from github.

Motivation and context

The main purpose is to enable subsequent verification of the app through AppVerifier, including adding the fingerprint to its internal database.

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/tchapgouv/tchap-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/tchapgouv/tchap-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()