Add README with small amount of documentation

README.md

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+# zkv
+
+Simple key-value store for single-user applications.
+
+## Pros
+
+* Simple file structure
+* Internal compression
+* Threadsafe operations through `sync.RWMutex`
+
+## Cons
+
+* Index stored in memory (`map[key hash (28 bytes)]file offset (int64)`)
+* Need to read the whole file on store open to create file index
+* No way to recover disk space from deleted records
+* Write/Delete operations block Read and each other operations
+
+## Usage
+
+Create or open existing file:
+
+```go
+db, err := Open("path to file")
+```
+
+Data operations:
+
+```go
+// Write data
+err = db.Set(key, value) // key and value can be any of type
+
+// Read data
+var value ValueType
+err = db.Get(key)
+
+// Delete data
+err = db.Delete(key)
+```
+
+## File structure
+
+Record is `encoding/gob` structure:
+
+| Field      | Description                        | Size     |
+| ---------- | ---------------------------------- | -------- |
+| Type       | Record type                        | uint8    |
+| KeyHash    | Key hash                           | 28 bytes |
+| ValueBytes | Value gob-encoded bytes            | variable |
+
+File is log stuctured list of commands:
+
+| Field  | Description              | Size     |
+| -------| ------------------------ | -------- |
+| Length | Record body bytes length | int64    |
+| Body   | Gob-encoded record       | variable |