Make pcap-broker go installable (#8)

This mainly adds a main.go file in the project root so it's easier to
install pcap-broker using `go install`:

    $ go install github.com/fox-it/pcap-broker@latest

Or install the main branch:

    $ go install github.com/fox-it/pcap-broker@main

README.md

@@ -15,10 +15,16 @@ More information on PCAP-over-IP can be found here:
 
 ## Building
 
-To build `pcap-broker`:
+Building `pcap-broker` requires the `libpcap` development headers, on Debian you can install it with:
 
 ```shell
-$ go build ./cmd/pcap-broker
+$ apt install libpcap-dev
+```
+
+To build from source, clone this repository and run:
+
+```shell
+$ go build .
 $ ./pcap-broker --help
 ```
 
@@ -29,6 +35,13 @@ $ docker build -t pcap-broker .
 $ docker run -it pcap-broker --help
 ```
 
+Alternatively, install directly using `go`:
+
+```shell
+$ go install github.com/fox-it/pcap-broker@latest
+$ pcap-broker --help
+```
+
 ## Running
 
 ```shell