build: restore README file

Author: thebaptiste

.metwork-framework/README.md

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+## What is it ?
+
+This is a [jinja2](http://jinja.pocoo.org/) extension to access to system
+environment variables. It is usefull if you have dynamically generated
+variable names.
+
+## Syntax
+
+The syntax is `{% raw %}{{ 'ENV_VAR'|getenv }}{% endraw %}` to access to the `ENV_VAR` environment
+variable. Don't forget the quotes around `ENV_VAR` !
+
+If you want to provide a default value to avoid an exception if the corresponding
+environment variable does not exist, you can use the following syntax:
+`{% raw %}{{ 'ENV_VAR'|getenv('default_value') }}{% endraw %}`.
+
+## Examples
+
+```python
+
+from jinja2 import Template, Environment
+
+# We load the extension in a jinja2 Environment
+env = Environment(extensions=["jinja2_getenv_extension.GetenvExtension"])
+
+# For the example, we use a template from a simple string
+template = env.from_string("the value of HOME environment variable is: "
+                           "{% raw %}{{ 'HOME'|getenv }}{% endraw %}")
+result = template.render()
+
+# [...]
+```
+