Add README to sample go module

# docker build . --rm --build-arg version=2.0.4 --build-arg commit=master -t heroiclabs/nakama:2.0.4

FROM golang:1.10-alpine3.7 as builder

FROM golang:1.11-alpine3.7 as builder

ARG commit

# docker build . --file ./plugin.Dockerfile --build-arg src=sample_go_module

# docker build . --file ./build/plugin.Dockerfile --build-arg src=sample_go_module

FROM golang:1.10-alpine3.7 as builder

FROM golang:1.11-alpine3.7 as builder

ARG src

# Nakama Go Runtime

Nakama Go Runtime uses the [Go Plugin](https://golang.org/pkg/plugin/) package to run custom server logic. If you've used the existing [Nakama Lua Runtime](https://heroiclabs.com/docs/runtime-code-basics/), then it should be pretty straight forward to use the Go Runtime as the APIs are very similar.

You can find more information about the Nakama Go Runtime on the [website](https://heroiclabs.com/docs/runtime-code-basics/).

## Plugin depedencies

You'll need to ensure that you have `heroiclabs/nakama/rtapi` and `heroiclabs/nakama/runtime` available in your `GOPATH`. You can then begin to implement the neccessary function that Nakama needs to interact with your plugin. Ensure that you have implemented the `InitModule` function as Nakama uses this as the entry point to your plugin.

You can find more information about the Nakama Go Runtime on the [website](https://heroiclabs.com/docs/runtime-code-basics/).

## Build plugin

### Binary

If you are running Nakama as a [binary](https://heroiclabs.com/docs/install-binary/), then you can easily build your plugin and instruct Nakama to load the shared object.

To do so, simply run the following command to build the plugin:

```

go build -buildmode=plugin

```

This will produce a `.so` file (platform dependant) that you'll need to place in the [modules directory](https://heroiclabs.com/docs/runtime-code-basics/#load-modules) of Nakama. You can then [start Nakama](https://heroiclabs.com/docs/install-start-server/#start-nakama) as usual.

### Docker 

If you are running Nakama through the [Docker image](https://heroiclabs.com/docs/install-docker-quickstart/), you'll need to also compile the plugin via the provided docker builder recipe. This is a known limitation in the Go compile toolchain as it cannot cross-compile plugins.

Download the content of the `plugin.Dockerfile` onto your system and run the following command:

```

docker build . --file ./plugin.Dockerfile --build-arg src=<ModuleName>

```

Make sure that you replace `<ModuleName>` with the absolute path to your plugin source file. Docker will then compile your plugin. To load the compiled plugin, you'll need to extract the shared object from the docker container:

cp sample_go_module.so ../data/modules/sample_go_module.so

```

docker run --rm --entrypoint cat <ContainerId> /go/build/<ModuleName>.so > ~/Desktop/<ModuleName>.so

```

Make sure that you change the `<ContainerId>` with the actual container identifier. You can do this by running

```

docker images

```

The compiled shared object file is on your desktop. You can move the shared object to a folder that you've assigned as your runtime module folder. Have a look at [this](https://heroiclabs.com/docs/install-start-server/#lua-modules) for more info.