docs: Update all the environment variables used (aws-powertools#127)

Author: pankajagrawal16

README.md

@@ -2,7 +2,7 @@
 
 ![aws provider](https://img.shields.io/badge/provider-AWS-orange?logo=amazon-aws&color=ff9900)
 
-A suite of utilities for AWS Lambda Functions that makes tracing with AWS X-Ray, structured logging and creating custom metrics asynchronously(Coming soon!) easier. ([AWS Lambda Powertools Python](https://github.com/awslabs/aws-lambda-powertools-python) is also available).
+A suite of utilities for AWS Lambda Functions that makes tracing with AWS X-Ray, structured logging and creating custom metrics asynchronously easier. ([AWS Lambda Powertools Python](https://github.com/awslabs/aws-lambda-powertools-python) is also available).
 
 **[📜Documentation](https://awslabs.github.io/aws-lambda-powertools-java/)** | **[Feature request](https://github.com/awslabs/aws-lambda-powertools-java/issues/new?assignees=&labels=feature-request%2C+triage&template=feature_request.md&title=)** | **[🐛Bug Report](https://github.com/awslabs/aws-lambda-powertools-java/issues/new?assignees=&labels=bug%2C+triage&template=bug_report.md&title=)** | **[Detailed blog post](https://aws.amazon.com/blogs/opensource/simplifying-serverless-best-practices-with-lambda-powertools/)**
 

docs/content/index.mdx

@@ -75,6 +75,17 @@ And configure the aspectj-maven-plugin to compile-time weave (CTW) the aws-lambd
 ```
 **Note:** If you are working with lambda function on runtime post java8, please refer [issue](https://github.com/awslabs/aws-lambda-powertools-java/issues/50) for workaround
 
+## Environment variables
+
+**Environment variables** used across suite of utilities.
+
+Environment variable | Description | Utility
+------------------------------------------------- | --------------------------------------------------------------------------------- | --------------------------------------------------------------------------------- | -------------------------------------------------
+**POWERTOOLS_SERVICE_NAME** | Sets service name used for tracing namespace, metrics dimension and structured logging | All
+**POWERTOOLS_METRICS_NAMESPACE** | Sets namespace used for metrics | [Metrics](./core/metrics)
+**POWERTOOLS_LOGGER_SAMPLE_RATE** | Debug log sampling | [Logging](./core/logging)
+**LOG_LEVEL** | Sets logging level | [Logging](./core/logger)
+
 ## Tenets
 
 * **AWS Lambda only** – We optimise for AWS Lambda function environments and supported runtimes only. Utilities might work with web frameworks and non-Lambda environments, though they are not officially supported.