Improve developer docs

[sbliven]

I tried to make some small fixes and came away frustrated with building the plugin. I'm familiar with typescript and npm so I expected this to be as simple as npm run build. However, the main branch (a8aa7e) fails to build currently, and without a CONTRIBUTING file or other developer-oriented documentation it wasn't obvious whether this was a problem with my installation or with the code.

I suggest adding CONTRIBUTING.md file with the following:

• Commands to build the software
• How to install a dev plugin in obsidian (eg with symlinks, possibly using helper plugins like hot-reload)
• What is expected of pull requests (eg pass lint)

You have CI actions set up, so enforcing that main builds and passes lint should be achievable.