add build section to docs

_docs/build.md

@@ -0,0 +1,20 @@
+# Build
+
+To build `ctop` from source, ensure you have a recent version of [glide](https://github.com/Masterminds/glide) installed and run:
+
+```bash
+go get github.com/bcicen/ctop && \
+cd $GOPATH/src/github.com/bcicen/ctop && \
+make build
+```
+
+To build a minimal Docker image containing only `ctop`:
+```bash
+make image
+```
+
+Now you can run your local image:
+
+```bash
+docker run -ti --name ctop --rm -v /var/run/docker.sock:/var/run/docker.sock ctop
+```