Add a proper README.md for this project. (#32)

Include diskUsage notes.

README.md

@@ -1,5 +1,72 @@
-# flatfs
-flatfs is a simple datastore implementation inspired by gits on-disk object
-storage.
+# go-ds-flatfs
 
+[![](https://img.shields.io/badge/made%20by-Protocol%20Labs-blue.svg?style=flat-square)](http://ipn.io)
+[![](https://img.shields.io/badge/project-IPFS-blue.svg?style=flat-square)](http://ipfs.io/)
+[![](https://img.shields.io/badge/freenode-%23ipfs-blue.svg?style=flat-square)](http://webchat.freenode.net/?channels=%23ipfs)
+[![standard-readme compliant](https://img.shields.io/badge/standard--readme-OK-green.svg?style=flat-square)](https://github.com/RichardLitt/standard-readme)
+[![GoDoc](https://godoc.org/github.com/ipfs/go-ds-flatfs?status.svg)](https://godoc.org/github.com/ipfs/go-ds-flatfs)
+[![Build Status](https://travis-ci.org/ipfs/go-ds-flatfs.svg?branch=master)](https://travis-ci.org/ipfs/go-ds-flatfs)
+[![Coverage Status](https://img.shields.io/codecov/c/github/ipfs/go-ds-flatfs.svg)](https://codecov.io/gh/ipfs/go-ds-flatfs)
 
+
+> A datastore implementation using sharded directories and flat files to store data
+
+`go-ds-flatfs` is used by `go-ipfs` to store raw block contents on disk. It supports several sharding functions (prefix, suffix, next-to-last/*).
+
+## Table of Contents
+
+- [Install](#install)
+- [Usage](#usage)
+- [Contribute](#contribute)
+- [License](#license)
+
+## Install
+
+`go-ds-flatfs` can be used like any Go module:
+
+
+```
+import "github.com/ipfs/go-ds-flatfs"
+```
+
+`go-ds-flatfs` uses [`Gx`](https://github.com/whyrusleeping/gx) and [`Gx-go`](https://github.com/whyrusleeping/gx-go) to handle dependendencies. Run `make deps` to download and rewrite the imports to their fixed dependencies.
+
+## Usage
+
+Check the [GoDoc module documentation](https://godoc.org/github.com/ipfs/go-ds-flatfs) for an overview of this module's
+functionality.
+
+### DiskUsage and accuracy
+
+This datastore implements the [`PersistentDatastore`](https://godoc.org/github.com/ipfs/go-datastore#PersistentDatastore) interface. It offers a `DiskUsage()` method which strives to find a balance between accuracy and performance. This implies:
+
+* The total disk usage of a datastore is calculated when opening the datastore
+* The current disk usage is cached frequently in a file in the datastore root (`diskUsage.cache` by default). This file is also
+written when the datastore is closed.
+* If this file is not present when the datastore is opened:
+  * The disk usage will be calculated by walking the datastore's directory tree and estimating the size of each folder.
+  * This may be a very slow operation for huge datastores or datastores with slow disks
+  * The operation is time-limited (5 minutes by default).
+  * Upon timeout, the remaining folders will be assumed to have the average of the previously processed ones.
+* After opening, the disk usage is updated in every write/delete operation.
+
+This means that for certain datastores (huge ones, those with very slow disks or special content), the values reported by
+`DiskUsage()` might be reduced accuracy and the first startup (without a `diskUsage.cache` file present), might be slow.
+
+If you need increased accuracy or a fast start from the first time, you can replace the `diskUsage.cache` file (while the
+datastore is not open), with the right disk usage value in size. I.e., in the datastore root:
+
+    $ du -sb .
+    3919232394        .
+    $ echo -n "3919232394" > diskUsage.cache
+
+
+## Contribute
+
+PRs accepted.
+
+Small note: If editing the README, please conform to the [standard-readme](https://github.com/RichardLitt/standard-readme) specification.
+
+## License
+
+MIT © Protocol Labs, Inc.