improv: add project tenets

Powertools now have official tenets that will guide us as we move towards GA. Tenets will also help us remain consistent across different language implementations as we grow.

Author: heitorlessa

Diff for: README.md

@@ -4,6 +4,17 @@
 
 A suite of utilities for AWS Lambda Functions that makes tracing with AWS X-Ray, structured logging and creating custom metrics asynchronously easier.
 
+## Tenets
+
+* **AWS Lambda only** – We optimize for AWS Lambda functions environment only. Utilities might work with web frameworks, and non-Lambda environments though they are not officially supported.
+* **Eases the adoption of best practices** – Utilities’ main priority is to facilitate best practices adoption defined in AWS Well-Architected Serverless Lens; everything else is optional.
+* **Keep it lean** – Additional dependencies are carefully considered to ease maintenance, security, and to prevent negatively impacting startup time. 
+* **We strive for backwards compatibility** – New features and changes should keep backwards compatibility. If a breaking change cannot be avoided, the deprecation and migration process should be clearly defined.
+* **We work backwards from the community** – We aim to strike a balance of what would
+work for 80% of customers. Emerging practices are considered and discussed via request for 
+comments (RFCs)
+* **Idiomatic** – Utilities follow language’s idioms and their best practices.
+
 ## Powertools available
 
 * [Python - Beta](./python/README.md)