add readme with instructions

docs/env-vars.md

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+# Environment variables management
+
+As a developer you will need to extend the current env vars for a service.
+
+The following rules must be followed:
+
+1. for each service that requires it, add it to the `services/docker-compose.yml` file (such as `MY_VAR=${MY_VAR}`)
+2. add a meaningful default value for development inside `.env-devel` so that developers can work, and that `osparc-simcore` is **self-consistent**.
+  - **NOTE** if the variable has a default inside the code, put the same value here
+3. inside the repo where devops keep all the secrets follow the instructions to add the new env var