Add manual installation instructions for latest Go releases to README.md

[sylvainfaivre]

The Manual Installation for macOS and Linux contained only the older bash scripts, so add instructions for the Go releases.

[ahmetb]

It would be much better to provide replace old instructions with a wget/curl + chmod that downloads binaries, or just an explanation saying "go to Releases page, and put the binary somewhere in your PATH and chmod+x it".

Honestly not sure how many of the users aren't able to figure this out themselves though.

[sylvainfaivre]

Well this MR's purpose was to give up to date instructions for the Go version.

When I installed this tool, I blindly followed the instructions from the README and installed the bash scripts.

Then a few months later, I wanted to see if there was an updated release, so I read the README again and was confused when I saw that I had in fact installed a much older release.

I feel that this MR fixes the instructions by providing an install procedure for the current version and making it clear that the bash scripts are the older version, and adding information about the completion scripts beeing needed in addition to the go binaries.

[ahmetb]

Readme changed significantly since then. Closing.