Document `--values-for` valid keys

[boweeb]

For the life of me, I can't find where valid keys are defined or how they're determined. This information would be particularly handy in the awless show --help output. FWIW, my immediate use-case is only concerned with EC2 instance "Launch Time".

awless show i-foobar shows an Uptime key but that doesn't work.

$ awless show --values-for Uptime i-foobar
[error]   no values for ["Uptime"]

aws-cli shows a LaunchTime key but that doesn't work either.

$ aws ec2 describe-instances --instance-ids=i-foobar | jq -r '.Reservations[].Instances[]'
{
  "AmiLaunchIndex": 0,
  "ImageId": "ami-foobaz",
  "InstanceId": "i-foobar",
  "InstanceType": "m4.large",
  "KeyName": "foo",
  "LaunchTime": "2020-04-03T16:10:26+00:00",
...

$ awless show --values-for LaunchTime i-foobar
[error]   no values for ["LaunchTime"]

My google-fu can only find examples of ID, name, and PublicIP. As an aside, the argument appears to be case-insensitive but that's not called out either.

[simcap]

To save you some time, and I am sorry to say that this project is no longer maintained unfortunately.

[boweeb]

@simcap Lol -- thanks for the heads up!