Add usage instructions to README

Author: carlzogh

README.md

@@ -25,6 +25,35 @@ Committed alongside the Dockerfiles in the branches are the tarballs, which ball
 
 Although we force-push the files away, the older versions of our images remain present on DockerHub and Amazon ECR.
 
+## Usage
+
+### Requirements
+To re-create the AWS Lambda base images, make sure you have the following pre-requisites set up:
+- [git](https://git-scm.com/downloads)
+- [git-lfs](https://git-lfs.github.com/)
+- [docker](https://docs.docker.com/get-docker/)
+
+### Building an image
+First, clone this repository:
+```
+git clone https://github.com/aws/aws-lambda-base-images
+```
+
+Then, checkout the branch relevant to the Lambda base image you want to build.
+
+eg. to build the `nodejs12.x` image, start by checking out the `nodejs12.x` branch:
+```
+git checkout nodejs12.x
+```
+
+Finally you can build your image as such:
+```
+docker build -t nodejs12.x:local -f Dockerfile.nodejs12.x .
+```
+
+This will use the Dockerfile at `Dockerfile.nodejs12.x` and tag the newly-built image as `nodejs12.x:local`.
+
+
 ## Licence
 
 This project is licensed under the Apache-2.0 License.