Add cache key elimination example

[huozhi]

Related to #94

Tested with codesandbox demo

[vercel]

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[vercel-team]

📝Changed routes:

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Commit 904c8eb (https://swr-site-inhcn7b3p.vercel.sh).

[shuding]

Maybe we should link the Least Recently Used Map text to https://github.com/rsms/js-lru?

Q1: To make it simpler, can we just createCache(new LRUMap(8))?
Q2: This is more like a concern, it's possible to have an active key removed (so it might be possible to cause some internal logic incorrectness?). A safe impl might be using ref counting like vercel/swr#92. Maybe let's hide this example away for now, and provide an example for clearing the cache during testing (with Jest). WDYT?

[huozhi]

@shuding sounds good to me, real-world inactive key detection would be much more complicated and not easy to demo in few lines.

[shuding]

Yeah, I think the more common cases are:

• clear cache after each test case/suit
• clear cache after logging out

Both cases are clearing the cache entirely. I don't see much value of selectively deleting cache values for saving memory for some JSON data.

After almost 3 years since open sourcing this lib we see no report on OOM caused by SWR, so I'm pretty sure this (automatic GC) is just the red herring and not a real demand (of course people can still do it if they want).

[huozhi]

Close this one as it's not sth we'd like user to focus on. Provide