Improve download and usage examples in README.md.
Improve download and usage examples in README.md.

Addresses #239. The `go get` command fetches the command-line tool, and does
not match the import path for the library. To make things more clear:

1. Mention explicitly that `go get` fetches the CLI. Also, to avoid potential
   issues with pre-modules Go versions, do the fetch in /tmp.

2. Include an import path explicitly in the source examples.

3. Mention explicitly how to import enry into a modules build.

Signed-off-by: M. J. Fromberger <b3372a9380d46b1c1dbc1f684b22f0fea1832403@gmail.com>

go-enry/go-enry,go-enry/go-enry,go-enry/go-enry,go-enry/go-enry,go-enry/go-enry,go-enry/go-enry,go-enry/go-enry