Update installation & contribution instructions & add Makefile

[radeksimko]

This change effectively enforces developers to build through the make target
and prevents server of version 0.0.0 from ever appearing in the wild.

As for the makefile logic:

• if the version is not provided we source the latest one from GitHub API
• in the "happy path" (being online and GitHub's API working correctly) make build "just works"
• otherwise (e.g. when offline) we can still build e.g. via VERSION=0.1.0 make build

[tommyknows]

Sorry for chiming in here, but I'm not quite sure why this has been introduced.
This change prevents people from just installing the language server through go get:

$> go get github.com/hashicorp/terraform-ls
$> terraform-ls
panic: Please use 'make build' to compile and install

goroutine 1 [running]:
github.com/hashicorp/terraform-ls/version.init.0()
        /Users/ramon/Documents/go/pkg/mod/github.com/hashicorp/terraform-ls@v0.2.0/version/version.go:27 +0xc5

I don't see why a server version 0.0.0 should explicitly be avoided, could you elaborate?
It would be nice if this project was just go-gettable like pretty much every other Go project.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the context necessary to investigate further.