Add recommendation for event usage and firing

For usage of events have a TLDR.

• wire-up to local DOM via @some-event in template (let Lit do the actual wire-up)
• wire-up events to this in constructor (no need to clean up - as it will be garbage collected on destroying)
• wire-up to elements outside of scope (ex. window) using connectedCallback , and remove them in disconnectedCallback.

For firing events have a TLDR.

• prefer “just” events without bubble or composed e.g. new Event('something-happened')
• when needing data consider extending Event or using CustomEvent
• use bubble: true if a node up the tree needs the event
• retrain from using composed: true as it leads to event pollution
• use composed only if absolutely needed

Add a link for details too https://dev.to/open-wc/composed-true-considered-harmful-5g59

[thepassle]

related/the same: https://github.com/open-wc/open-wc/issues/895#issuecomment-554067855

[thepassle]

Maybe even a codelab?