added some documentation for AWSConfig

[ExpandingMan]

I needed to hack the request URL for using a local instance of min.io, and I was afriad I would have to change a bunch of stuff support that, but I was happy to discover that this functionality is already supported, at least in AWSS3.jl.

I've added some documentation to the string to make this slightly easier to discover.

Note that I'm not actually 100% sure that every service supports this. The functionality that was actually relevant to me was a check for :endpoint in AWSS3.jl, so at this point there would seem to be nothing AWSCore that provides this for services in a generic way.

If there are more available options that I'm unaware of, please let me know and I'll add to this string, thanks!

[mattBrzezinski]

I believe these are all that are used, you can add your own keys into the SymDict yourself if you want.

Do you have any documentation for setting up Minio? It's something which I've wanted to setup for testing S3 in #100

[ExpandingMan]

Min.io clearly went to great lengths to be compatible with S3 stuff, the only trick really is that the host part of the URL has to change, and this is done rarely enough that not everything will support that. boto3 seems like it does since the CLI supports it via --endpoint-url. I was kind of expecting we wouldn't support it yet, but to my pleasant surprise AWSS3 at least does indeed. So all you need to do to use min.io is just e.g. AWSConfig(:endpiont=>"http://localhost:9000", ...) and that's it, it should work.

Unfortunately the newly overhauled AWS.jl does not support the URL swapping, but I plan to make a PR to it soon unless there is some roadblock to doing that that I'm not currently seeing.

Anyway, when I'm done messing with all this I'll try to document it somewhere. Maybe a discourse post or something.