Added Silverbullet note taking app

[sohmc]

Thanks for taking the time to suggest an addition to awesome-selfhosted!

To ensure your Pull Request is dealt with swiftly, please check the following (check the boxes [x]):

• [x] Submit one item per pull request. This eases reviewing and speeds up inclusion.
• [x] You have searched the repository for any relevant issues or PRs, including closed ones.
• [x] Any software you are adding is not already listed at any of awesome-sysadmin, staticgen.com, staticsitegenerators.net, dbdb.io.
• [x] The file you are adding is formatted as described in addition.md.
• [x] Demo links should only be used for interactive demos, i.e. not video demonstrations.
• [x] Comments and unused optional fields have been removed.
• [x] The file you are adding uses kebab-case file naming, for example my-awesome-software.yml.
• [x] Values for platform are the main server-side requirements for the software. Don't include frameworks or specific dialects.
• [x] Any software project you are adding to the list is actively maintained.
• [x] Any software project you are adding was first released more than 4 months ago.
• [x] Any software project you are adding has working installation instructions.
• [x] You understand that your Pull Request will be merged ~1 week after approval, to allow for further comments if needed.

[sohmc]

I am pretty sure I got everything correct. Please let me know if there is information missing.

Please note: I have not personally used the software and my inclusion is not an endorsement, but a way to help add info to the existing list.

[sohmc]

I pushed the suggested changes to the PR. I apologize for the delay. I've been traveling.