Add instructions how to install or publish

[iilyak]

This extension look really cool and somewhat similar in spirit with non VScode based tools:

• imdone/imdone-core: Text based kanban processor
• entozoon/readme-kanban-board: Write a markdown based kanban board and have it converted to a nice image in your readme

However I couldn't find instructions how to install it to VSCode (I use https://github.com/VSCodium/vscodium) I tried to extract it into my local directory to side load. However it didn't appear to be installed:

git clone https://github.com/brokensandals/markheadboard $HOME/.vscode-oss/extensions/brokensandals.markheadboard-1.2.0

[brokensandals]

Normally, you'd install via the VSCode extension marketplace. I hadn't heard of VSCodium before but it looks like they have a separate registry - when I get time to read more about it I'll try to upload the extension there too.

In the meantime, if you go to the extension's page in the VS Marketplace and find the "Download Extension" link, that should let you download a .vsix file, and the VSCodium docs seem to indicate you can install using that. I think you'd open the command palette, find "Extensions: Install from VSIX", then select the file.