Improve documentation for spec-compliant caches

[ungive]

The cache that needs to be in front of the loon server has many requirements:

• It must forward requests synchronously (don't request again, when a request for the same content is already ongoing)
• It must not cache any query parameters, as these are not supported and could be used to bypass the cache
• Cache-Control request headers must be ignored, otherwise the cache can be bypassed
• The content must be cached for the configured duration, starting at the point in time at which the content was fully committed to the cache, i.e. the cache should not be invalidated too early for long running uploads (obvious, but worth mentioning)
  See Requirements for cache duration and long-running uploads #18

It would be very helpful to document several spec-compliant caches and provide well-documented deployment examples for each.