Add some instructions to create contributing development environment.

contributing.Dockerfile

@@ -0,0 +1,5 @@
+FROM rust
+# Install nmap first.
+RUN apt-get update -qy && apt-get install -qy nmap
+# Then install rustfmt and clippy for cargo.
+RUN rustup component add rustfmt clippy

contributing.md

@@ -29,3 +29,38 @@ If you have any feature suggestions or bugs, leave a GitHub issue. We welcome an
 
 ## Rewarding you
 I cannot pay you :-( But, I can place your GitHub profile on the README under `#Contributors` as a thank you! :)
+
+## Contributing development environment
+
+To ease contribution to RustScan, you can use the `contributing.Dockerfile` to create a Docker image ready to build and play with RustScan.
+To build it you just need to run:
+
+```bash
+you@home:~/RustScan$ docker build -t rustscan_contributing -f contributing.Dockerfile
+```
+
+Then you need to run the container with a volume so it can access, *with read and write permissions*, to RustScan files:
+
+```bash
+you@home:~/RustScan$ docker run -ti --rm -v "$PWD":/rustscan -w /rustscan rustscan_contributing bash
+```
+
+You can now modify RustScan files with your favorite editor, once you want to compile and test your modifications, type the following in the container prompt:
+
+```bash
+root@container:/rustscan# cargo build
+```
+
+You are now ready to use RustScan:
+
+```bash
+root@container:/rustscan# cargo run -- -b 2000 -t 5000 -a 127.0.0.1
+```
+
+You can also format, lint with `clippy` and test the code with the following commands:
+
+```bash
+root@container:/rustscan# cargo fmt
+root@container:/rustscan# cargo clippy
+root@container:/rustscan# cargo test
+```