*: add a go.mod file

[ericchiang]

Generated using the following commands:

go mod init github.com/coreos/go-oidc
go mod tidy

Fixes #218

NOTE: this is expected to fail due to travis issues until #227 is merged and this is rebased.

[mikedanese]

Should we bump these?

go-oidc/.travis.yml

Lines 4 to 5 in 2be1c5b

	- "1.9"
	- "1.10"

[mikedanese]

Each major Go release is supported until there are two newer major releases.

https://golang.org/doc/devel/release.html#policy

1.13, 1.12?

[ericchiang]

Bumped in #227 :)

[ericchiang]

I'll pull it out into this PR

[pjediny]

Should not this be /v2 or better /v3 module?

[ericchiang]

Happy to do that for later breaking changes, but I'd rather not force users to change the import path today.

[pjediny]

Would you not need to change it anyway from gopkg.in/coreos/go-oidc.v2 to github.com/coreos/go-oidc? I'm not sure how the go would work with this module, as they mandate the semver major change to be reflected by import paths. But I don't know how this works with gopkg.in, having special exception in module versioning.

[ericchiang]

This package doesn't use gopkg.in.

Though it does look like some people import it this way... https://godoc.org/gopkg.in/coreos/go-oidc.v2?importers

I'll open bugs against those importers.

[pjediny]

And my private project too :-), hence the confusion.
If I can suggest, maybe it should be documented in the README.md in a similar way it is done in github.com/square/go-jose.

[ericchiang]

This is documented in the godoc link, examples, and now the go.mod. I think that should be good :)