upload pre-buit binaries on releases

[Sylvain303]

following #23

• [x] provide a script to upload signed prebuit binaries on github releases.

hints are:

• make all on travis and upload to github on deploy step built locally

• locally with a script

• https://goreleaser.com/ ?

• [x] Also update getter script:

https://github.com/docopt/docopts/blob/master/get_docopts.sh

• [x] Document about prebuilt binaries
• [x] change version format: `docopts: 0.6.3, commit yxz1234, built at 2019-08-03 10:01:06 (UTC+0200)

[Sylvain303]

• documentation on pre-building binaries https://github.com/docopt/docopts/blob/master/docs/pre_built_binaries.md
• binaries are built in build/ folder with gox
• deployed with deploy.sh using gothub

To be continued.

[Sylvain303]

v0.6.3-rc1 published with our deploy.sh OK

I didn't use https://goreleaser.com/ for that release, I may try it in the future.

I also introduced ldflags with govvv See: https://github.com/docopt/docopts/blob/v0.6.3-rc1/Makefile#L16

Seems OK.