Update to use modules

Plugging-in-Plugins-Yourself.md

@@ -1,29 +1,24 @@
-You can build Caddy from source and plug in plugins without having to use the build server. You just need [Go installed](https://golang.org/doc/install). Then get this repo by running:
-
-```bash
-$ go get github.com/mholt/caddy/caddy
-```
-
-It will be saved in a subfolder of `$GOPATH/src`. (Default GOPATH is `$HOME/go`).
-
-Simply open `caddy/caddymain/run.go` and add an import at the top of the file, to the package of the plugin you're installing:
+You can build Caddy from source and plug in plugins without having to use the build server and without changing Caddy's source code! You need [Go installed](https://golang.org/doc/install).
 
+1. [Install Go](https://golang.org/doc/install) if you don't have it already.
+2. Set the transitional environment variable for Go modules: `export GO111MODULE=on`
+3. Create a new folder anywhere (doesn't have to be in `$GOPATH`) - naming it `caddy` is preferred.
+4. Put this Go file into it, modifying it to import the plugins you want:
 ```go
-	// This is where other plugins get plugged in (imported)
-	_ "your/plugin/package/path/here"
+package main
+
+import (
+	"github.com/mholt/caddy/caddy/caddymain"
+	
+	// plug in plugins here, for example:
+	// _ "import/path/here"
+)
+
+func main() {
+	// optional: disable telemetry
+	// caddymain.EnableTelemetry = false
+	caddymain.Run()
+}
 ```
-
-For example, to plug in the http.git plugin:
-
-
-```go
-	_ "github.com/abiosoft/caddy-git"
-```
-
-Then on the command line in that same folder, run:
-
-```bash
-$ go run build.go
-```
-
-The custom binary will be saved in that folder as `caddy` which you can run.
+5. Make your little main function a Go module: `go mod init mycaddy` (the name doesn't really matter).
+6. Then `go install` will then create your binary at `$GOPATH/bin`, or `go build` will put it in the current directory.