update Cargo.toml

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

mini-lsm/Cargo.toml

@@ -6,6 +6,7 @@ homepage = { workspace = true }
 keywords = { workspace = true }
 license = { workspace = true }
 repository = { workspace = true }
+description = "A tutorial for building an LSM tree storage engine in a week."
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html