Add README.md with development guidelines (#32)

* Create README.md

Author: Camille

README.md

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+# Openware docker images
+
+This repository contains docker images maintained by openware team. Mostly blockchain cryptonodes.
+You can find the list of available images and tag here: https://quay.io/organization/openware
+
+## Development guidelines
+
+We are focusing our efforts to keep a high level of trust and security on those images.
+Please follow the following principles when developing an image:
+
+1. Use official images as source (ex: debian:buster, alpine, python), don't use sources from third parties
+2. Install dependencies by downloading from official repositories (do not add them to the git reposity)
+3. Use recent versions
+4. The target application should be downloaded from well identified sources (ex: github official repository releases)
+5. Store the checksums of the binaries you install and check them during the build in the Dockerfile.
+   See the following example: [electrum/Dockerfile](./electrum/Dockerfile) and [electrum/checksum.sha256](./electrum/checksum.sha256)
+6. The application version must be stored in VERSION file, the content is passed as argument during the build and it's used to tag the image.
+   It **must be used to download the application binaries or sources**
+   You can build manually with the following command: ```docker build --build-arg VERSION=1.2.3```
+7. The image should contain a README.md with instructions to do basic operations
+8. Include a docker-compose.yml file with example how to deploy a testnet and mainnet node (add configuration files if necessary)
+9. Create volumes for data directories (blockchain storage and wallets)