Post SHA-256 hash of Developer Signing Key

[shrimprugbysnowowl]

The project has many users who have degoogled devices. As was mentioned in Issue 126, the project offers a direct apk download option from SpiritCroc as well as its own fdroid repo. Users who install the direct download apk should have a way to verify that the build was signed by the developer. Posting the sha-256 hash of the developer signing key in the project README and on the SpiritCroc website.

Based on the downloaded SchildiChat[f].apk, the hash appears to be `FA:C0:73:C4:2D:F8:A2:4E:D7:9E:D6:1A:19:86:A0:08:B6:3D:64:68:99:21:70:F8:2C:29:D7:9B:C6:80:8E:AC'. I'm happy to create a PR for the README if you are amenable to inclusion.

I greatly appreciate all the hard work on this project, and thank you for your consideration.

[SpiritCroc]

Sounds like a reasonable thing to add to the website. The README doesn't link direct downloads so doesn't make sense to add there for me. But you can create a PR for the website if you want: https://github.com/SchildiChat/website

SHA-256 hashes for builds in the main repo, as provided by keytool -printcert -jarfile from the build machine:

• SchildiChat [g] and [f] builds: FA:C0:73:C4:2D:F8:A2:4E:D7:9E:D6:1A:19:86:A0:08:B6:3D:64:68:99:21:70:F8:2C:29:D7:9B:C6:80:8E:AC
• Play-Store compatible builds: 66:12:AD:E7:E9:31:74:A5:89:CF:5B:A2:6E:D3:AB:28:23:1A:78:96:40:54:6C:8F:30:37:5E:F0:45:BC:92:42