Improve README documentation

README.md

@@ -1,13 +1,67 @@
 # gompress
 CLI tool to compress the contents of AWS S3 bucket.
 
-Until better documentation is available, download the code, build it using Go
-(1.13), and run the binary providing all the flags that the binary expects
-(source and destination).
+Gompress takes files from the original location, compresses them on the fly
+using gzip, and uploads to their destination, keeping the original names (only
+adding `.gz`).
 
-`AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` environment variables must be
-set, and they should belong to a user with sufficient permissions to list files
-in s3 bucket, download them, and upload them to the location provided in
-configuration.
+Gompress never keeps large files completely in memory, and never writes them to
+disk. It streams data from source into destination, adding a compression layer
+in the middle.
 
-Pre-compiled binaries, docker image and better docs are coming.
+It is recommended to become familiar with [AWS S3
+pricing](https://aws.amazon.com/s3/pricing/) before using this utility. Moving
+files around costs money. To reduce the cost, use EC2 instances in the same
+region as the buckets!
+
+## Usage
+
+### Running in a docker container:
+
+```
+docker run --rm \
+    -e AWS_ACCESS_KEY_ID \
+    -e AWS_SECRET_ACCESS_KEY \
+    orlangure/gompress \
+    -src-region us-east-1 \
+    -src-bucket uncompressed-files \
+    -src-prefix some-folder/and-sub-folder \
+    -dst-region us-east-1 \
+    -dst-bucket compressed-files \
+    -dst-prefix a-folder \
+    -keep
+```
+
+AWS credentials with sufficient permissions to list, get, create and delete
+objects are required. AWS key and secret should be provided using [environment
+variables](https://docs.docker.com/engine/reference/commandline/run/#set-environment-variables--e---env---env-file).
+
+When `-keep` flag is provided, original files remain in their original
+location. Otherwise, when `-keep` flag is not set, original files are removed
+once their compressed versions are successfully uploaded to the destination
+path.
+
+### Building from source (Go 1.13 is required):
+
+```
+$ git clone https://github.com/orlangure/gompress
+$ cd gompress
+$ go install .
+
+$ gompress
+Usage of gompress:
+  -dst-bucket string
+        target s3 bucket name
+  -dst-prefix string
+        new files will be prefixed with this value
+  -dst-region string
+        target region (default "us-east-1")
+  -keep
+        set to keep original files (remove by default)
+  -src-bucket string
+        source s3 bucket name
+  -src-prefix string
+        source file prefix
+  -src-region string
+        source region (default "us-east-1")
+```