docs: introduced README

README.md

@@ -19,6 +19,37 @@ HappyPancake to run real-time aggregation and long-term data analysis
 on the largest social website in Sweden. You can read more about it in
 [Real-time Analytics with Go and LMDB](https://abdullin.com/bitgn/real-time-analytics/).
 
+# Design
+
+Cellar stores data in a very simple manner:
+
+- LMDB database is used for keeping metadata (including user-defined);
+- a single pre-allocated file is used to buffer all writes;
+- when buffer fills, it is compressed, encrypted and added to the chunk list.
+
+You can have **only one writer at a time**. This writer has two operations:
+
+- `Append` - adds new bytes to the buffer, but doesn't flush it.
+- `Checkpoint` - performs all the flushing and saves the checkpoints.
+
+The store is optimized for throughput. You can efficiently execute
+thousands of appends followed by a single call to `Checkpoint`.
+
+Whenever a buffer is about to overflow (exceed the predefined max
+size), it will be "sealed" into an immutable chunk (compressed,
+encrypted and added to the chunk table) and replaced by a new buffer.
+
+See tests in `writer_test.go` for sample usage patters (for both
+writing and reading).
+
+
+At any point in time multiple readers could be created, they will be
+working against the last available snapshot, optionally reading into
+the buffer as well. Readers have only one operation available for
+them - reading sequentially within a given interval (or scanning
+through the entire storage).
+
+
 # License
 
 3-clause BSD license.