Add S3 gateway documentation (#6165)

Fixes #4830

docs/gateway/s3.md

@@ -0,0 +1,72 @@
+# Minio S3 Gateway [![Slack](https://slack.minio.io/slack?type=svg)](https://slack.minio.io)
+
+Minio S3 Gateway adds Minio features like Minio Browser and disk caching to AWS S3 or any other AWS S3 compatible service.
+
+## Run Minio Gateway for AWS S3
+
+As a prerequisite to run Minio S3 gateway, you need valid AWS S3 access key and secret key.
+
+### Using Docker
+
+```
+docker run -p 9000:9000 --name minio-s3 \
+ -e "MINIO_ACCESS_KEY=aws_s3_access_key" \
+ -e "MINIO_SECRET_KEY=aws_s3_secret_key" \
+ minio/minio gateway s3
+```
+
+### Using Binary
+
+```
+export MINIO_ACCESS_KEY=aws_s3_access_key
+export MINIO_SECRET_KEY=aws_s3_secret_key
+minio gateway s3
+```
+
+## Run Minio Gateway for AWS S3 compatible services
+
+As a prerequisite to run Minio S3 gateway on an AWS S3 compatible service, you need valid access key, secret key and service endpoint.
+
+### Using Docker
+
+```
+docker run -p 9000:9000 --name minio-s3 \
+ -e "MINIO_ACCESS_KEY=access_key" \
+ -e "MINIO_SECRET_KEY=secret_key" \
+ minio/minio gateway s3 --address https://s3_compatible_service_endpoint:port
+```
+
+### Using Binary
+
+```
+export MINIO_ACCESS_KEY=access_key
+export MINIO_SECRET_KEY=secret_key
+minio gateway s3 --address https://s3_compatible_service_endpoint:port
+```
+
+## Minio Caching
+
+Minio edge caching allows storing content closer to the applications. Frequently accessed objects are stored in a local disk based cache. Edge caching with Minio gateway feature allows
+
+- Dramatic improvements for time to first byte for any object.
+- Avoid S3 [data transfer charges](https://aws.amazon.com/s3/pricing/).
+
+Refer [this document](https://docs.minio.io/docs/minio-disk-cache-guide.html) to get started with Minio Caching.
+
+## Minio Browser
+
+Minio Gateway comes with an embedded web based object browser. Point your web browser to http://127.0.0.1:9000 to ensure that your server has started successfully.
+
+![Screenshot](https://github.com/minio/minio/blob/master/docs/screenshots/minio-browser-gateway.png?raw=true)
+
+With Minio S3 gateway, you can use Minio browser to explore AWS S3 based objects.
+
+### Known limitations
+
+- Bucket notification APIs are not supported.
+
+## Explore Further
+
+- [`mc` command-line interface](https://docs.minio.io/docs/minio-client-quickstart-guide)
+- [`aws` command-line interface](https://docs.minio.io/docs/aws-cli-with-minio)
+- [`minio-go` Go SDK](https://docs.minio.io/docs/golang-client-quickstart-guide)