Add Sustainable Software Development guidelines link

Added link to Sustainable Software Development guidelines.

Author: michael-s-ashida

README.md

@@ -108,6 +108,7 @@ Do you know of an incredible green software repository that we missed? Let us kn
 
 ## Special Tools
 - [Patterns](https://github.com/Green-Software-Foundation/patterns): An online open-source database of software patterns reviewed and curated by the Green Software Foundation across a wide range of categories.
+- [Sustainable-software-guidelines](https://github.com/sustainable-software-guidelines/Sustainable-Software-Development): Guidelines for developing energy-efficienct and sustainable software, developed by the Green Computing Pillar of the Green Digital Action Initiative. 
 - [Nvidia-co2](https://github.com/kylemcdonald/nvidia-co2): Show gCO2eq emissions information with nvidia-smi, at the top right corner. For example: 79.2gCO2eq/h or 23.76mm^2/h sea ice.
 - [Sustainability-scanner](https://github.com/awslabs/sustainability-scanner): Sustainability scanner is an open source tool that helps you create a more sustainable infrastructure on AWS. It takes in your Cloudformation template as input, evaluates it against a set of sustainability best practices and generates a report with a sustainability score and suggested improvements to apply to your template. 
 - [Leaf](https://github.com/dos-group/leaf): LEAF is a simulator for analytical modeling of energy consumption in cloud, fog, or edge computing environments. It enables the modeling of simple tasks running on a single node as well as complex application graphs in distributed, heterogeneous, and resource-constrained infrastructures. LEAF is based on SimPy for discrete-event simulation and NetworkX for modeling infrastructure or application graphs.