Add docs to implementation-notes.md about not needing to dispose much

[jbphet]

Related to code review, see #165.

There's a line item in the code review checklist that says, "Are there leaks due to registering observers or listeners? The following guidelines should be followed unless documentation (in-line or in implementation-notes.md) describes why following them is not necessary."

Very few of the link calls in the sim have accompanying unlink calls, but I think this is just the nature of the sim - there just aren't a lot of things coming and going. The only dynamic elements are the particles, and there is mention of them and their disposal functions in implemenation-notes.md. I'd recommend adding something to that same doc that says something like, "Other than particles, everything else pretty much stays around for the lifetime of the sim and therefore doesn't need to be unlinked or disposed".

To be clear, there's no need to add comments or change the code for this, as it's generally pretty clear why there aren't any unlinks.

[zepumph]

To be looped into #94

[zepumph]

• Maybe in this disposal section, also talk about how this.particles is like a "main list" and once it is removed from it, the particle is disposed.

[Luisav1]

Added in the commit above. Closing.