docs: add comments & hints for day one starter and reference code (#18)

* feat(docs): Improve/Add comments & some hints for day one starter code

* feat(docs): Add comments for day one solution code

* feat(docs): Add figure for block storage format in starter code (block.rs)

mini-lsm-starter/src/block.rs

@@ -5,21 +5,32 @@ mod builder;
 mod iterator;
 
 pub use builder::BlockBuilder;
+/// You may want to check `bytes::BufMut` out when manipulating continuous chunks of memory
 use bytes::Bytes;
 pub use iterator::BlockIterator;
 
-/// A block is the smallest unit of read and caching in LSM tree. It is a collection of sorted
-/// key-value pairs.
+/// A block is the smallest unit of read and caching in LSM tree.
+/// It is a collection of sorted key-value pairs.
+/// The `actual` storage format is as below (After `Block::encode`):
+///
+/// ----------------------------------------------------------------------------------------------------
+/// |             Data Section             |              Offset Section             |      Extra      |
+/// ----------------------------------------------------------------------------------------------------
+/// | Entry #1 | Entry #2 | ... | Entry #N | Offset #1 | Offset #2 | ... | Offset #N | num_of_elements |
+/// ----------------------------------------------------------------------------------------------------
 pub struct Block {
     data: Vec<u8>,
     offsets: Vec<u16>,
 }
 
 impl Block {
+    /// Encode the internal data to the data layout illustrated in the tutorial
+    /// Note: You may want to recheck if any of the expected field is missing from your output
     pub fn encode(&self) -> Bytes {
         unimplemented!()
     }
 
+    /// Decode from the data layout, transform the input `data` to a single `Block`
     pub fn decode(data: &[u8]) -> Self {
         unimplemented!()
     }