Update release instructions.

## Steps

To build releases for a variety of platforms we use the excellent [xgo](https://github.com/karalabe/xgo) project. You will need Docker engine installed. These steps should be followed from the project root folder.

To build releases for a variety of platforms we use the excellent [xgo](https://github.com/techknowlogick/xgo) project. You will need Docker engine installed. These steps should be followed from the project root folder.

These steps are one off to install the required build utilities.

1. Install the xgo Docker image.

   ```

   docker pull karalabe/xgo-latest

   docker pull techknowlogick/xgo:latest

   ```

2. Install the command line helper tool. Ensure "$GOPATH/bin" is on your system path to access the executable.

   ```

   env GO111MODULE=off go get -u github.com/karalabe/xgo

   env GO111MODULE=off go get -u src.techknowlogick.com/xgo

   ```

These steps are run for each new release.

   __Note__: In source control good semver suggests a "v" prefix on a version. It helps group release tags.

   ```

   git add CHANGELOG

   git add CHANGELOG.md

   git commit -m "Nakama 2.1.0 release."

   git tag -a v2.1.0 -m "v2.1.0"

   git push origin v2.1.0 master

3. Execute the cross-compiled build helper.

   ```

   xgo --targets=darwin/amd64,linux/amd64,linux/arm64,windows/amd64 --ldflags "-s -w -X main.version=2.1.0 -X main.commitID=$(git rev-parse --short HEAD 2>/dev/null)" github.com/heroiclabs/nakama

   xgo --targets=darwin/amd64,linux/amd64,linux/arm64,windows/amd64 --trimpath --ldflags "-s -w -X main.version=2.1.0 -X main.commitID=$(git rev-parse --short HEAD 2>/dev/null)" github.com/heroiclabs/nakama

   ```

   This will build binaries for all target platforms supported officially by Heroic Labs.

4. Package up each release as a compressed bundle.

   ```

   tar -czf "nakama-<os>-<arch>" nakama README.md LICENSE CHANGELOG.md

   tar -czf "nakama-<os>-<arch>.tar.gz" nakama README.md LICENSE CHANGELOG.md

   ```

5. Create a new draft release on GitHub and publish it with the compressed bundles.