add preface

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-book/src/week1-overview.md

@@ -1,7 +1,17 @@
-# Overview
+# Week 1 Overview: Mini-LSM
 
 ![Chapter Overview](./lsm-tutorial/week1-overview.svg)
 
-key functionalities index
+In the first week of the tutorial, you will build necessary storage formats for the storage engine, the read path and the write path of the system, and have a working implementation of an LSM-based key-value store. There are 7 chapters (days) for this part.
+
+* Day 1: Memtable. You will implement the in-memory read and write path of the system.
+* Day 2: Merge iterator. You will extend what you have built in day 1 and implement a `scan` interface for your system.
+* Day 3: Block encoding. Now we start the first step of the on-disk structure and build the encoding/decoding of the blocks.
+* Day 4: SST encoding. SSTs are composed of blocks and at the end of the day, you will have the basic building blocks of the LSM on-disk structure.
+* Day 5: Read path. Now that we have both in-memory and on-disk structures, we can combine them together and have a fully-working read path for the storage engine.
+* Day 6: Write path. In day 5, the test harness generates the structures, and in day 6, you will control the SST flushes by yourself. You will implement flush to level-0 SST and the storage engine is complete.
+* Day 7: SST optimizations. We will implement several SST format optimizations and improve the performance of the system.
+
+At the end of the week, your storage engine should be able to handle all get/scan/put requests. The only missing parts are persisting the LSM state to disk and a more efficient way of organizing the SSTs on the disk. You will have a working *Mini-LSM* storage engine.
 
 {{#include copyright.md}}