docs: explain how to use multiple MFA virtual devices for same account

[kogitant]

Explains how to set up multiple "credential profiles" and "role profiles" to be able to use both Google Authenticator and Yubikey to provide TOTP for the same AWS account assuming the same target role.

Fixes #38

[kogitant]

If you have everything else set up correctly, but you have not added the [work-yubikey] "credential profile" to ~/.aws/credentials you will get an error like this:

vegas-credentials assume --verbose --profile=somerole@work-acc-2@y
ℹ️  Credentials: Role: =arn:aws:iam::2222222222222:role/somerole
🔧 Credentials: Cache: /Users/FrankSinatra/Library/Caches/vegas-credentials/session-cache
ℹ️  Credentials: Cache: Not found
🔧 Yubikey: OATH password cache: /Users/FrankSinatra/Library/Caches/vegas-credentials/yubikey-oath-access
🔧 Yubikey: Device Serial: 1234567890
🔧 Yubikey: OATH Account: Amazon Web Services:FrankSinatra@vegas-demo-account
ℹ️  Yubikey: device is available
🔒 Yubikey: OATH application is password protected
🔐 Yubikey: OATH password found from cache
🔑 MFA: Input Token (via CLI stdin / GUI prompt / Yubikey touch):
ℹ️  MFA: Token received via Yubikey touch
ℹ️  MFA: Token received: "123456"
ℹ️  MFA: Serial: arn:aws:iam::111111111111:mfa/FrankSinatra@virtual-yubikey-oath
panic: operation error STS: AssumeRole, exceeded maximum number of attempts, 3, failed to sign request: failed to retrieve credentials: failed to refresh cached credentials, no EC2 IMDS role found, operation error ec2imds: GetMetadata, exceeded maximum number of attempts, 3, request send failed, Get "http://169.254.169.254/latest/meta-data/iam/security-credentials/": dial tcp 169.254.169.254:80: connect: host is down

goroutine 1 [running]:
github.com/aripalo/vegas-credentials/internal/credentials.(*Credentials).New(0xc000330360, {0xc00044e6e0, 0x6})
/home/runner/work/vegas-credentials/vegas-credentials/internal/credentials/credentials.go:114 +0x8f1
github.com/aripalo/vegas-credentials/internal/app.(*App).Assume(0xc0006b3d38, {{0x7ff7bfeff58d?, 0x0?}})
/home/runner/work/vegas-credentials/vegas-credentials/internal/app/assume.go:56 +0x505
github.com/aripalo/vegas-credentials/cmd.glob..func3(0x1dfd900?, {0x181809b?, 0x2?, 0x2?})
/home/runner/work/vegas-credentials/vegas-credentials/cmd/assume.go:36 +0xcc
github.com/spf13/cobra.(*Command).execute(0x1dfd900, {0xc000147540, 0x2, 0x2})
/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.4.0/command.go:856 +0x67c
github.com/spf13/cobra.(*Command).ExecuteC(0x1dfdb80)
/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.4.0/command.go:974 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.4.0/command.go:902
github.com/aripalo/vegas-credentials/cmd.Execute()
/home/runner/work/vegas-credentials/vegas-credentials/cmd/cmd.go:74 +0x25
main.main()
/home/runner/work/vegas-credentials/vegas-credentials/main.go:25 +0xba