Add go.mod in order to allow semantic import versioning

[m90]

As the latest version is tagged as v4.x.x in git, using the package as a Go module will require importing it as github.com/go-telegram-bot-api/telegram-bot-api/v4, otherwise it will show up as:

github.com/go-telegram-bot-api/telegram-bot-api v4.6.2+incompatible
in a dependent's go.mod.

See: https://github.com/golang/go/wiki/Modules#semantic-import-versioning

[m90]

Looks like this is not what should be done right now: golang/go#25967 (comment)

[thepudds]

@m90 one quick comment: it looks like this package has been using gopkg.in as the standard import path for consumers?

If that is the case, then the whole complexity of semantic import versioning for v2+ Go modules is really side-stepped, because Go modules keep an import like import "gopkg.in/telegram-bot-api.v4" working just as it always has, and there is no requirement to change import paths from a working gopkg.in/telegram-bot-api.v4 if the project opts in to Go modules.

From the doc:
https://tip.golang.org/cmd/go/#hdr-Module_compatibility_and_semantic_versioning

As a special case, module paths beginning with gopkg.in/ continue to use the conventions established on that system: the major version is always present, and it is preceded by a dot instead of a slash: gopkg.in/yaml.v1 and gopkg.in/yaml.v2, not gopkg.in/yaml and gopkg.in/yaml/v2.

One way to look at it is this project already opted in to having the major version in its import paths by virtue of using gopkg.in, and Go modules know how to understand that properly (and of course older Go versions like 1.8, 1.9, 1.10 continue to work with gopkg.in just like they always have).

Opt-ing in to Go modules is reasonably straightforward for a v0 or v1 project, and it should be more-or-less just as straightforward to opt-in to Go modules for a v2+ project that was already publishing itself via gopkg.in...

All that said, Go modules are 100% opt-in at this point, so makes sense to do whatever works best for your project.

CC @myitcv in case he has any additional comments.