README.md: add content

README.md

@@ -5,3 +5,47 @@
 [![codecov](https://codecov.io/gh/invzhi/timex/branch/main/graph/badge.svg?token=I2M6JCGY84)](https://codecov.io/gh/invzhi/timex)
 
 📅 A Go package for working with date.
+
+## Why use timex?
+
+- Self-contained type `Date`, focusing on date-specific operations.
+- Don't rely on type `time.Time`, avoid timezone-related issues.
+- Represent date format like `YYYY-MM-DD` instead of `2006-01-02`.
+- Working with date type of MySQL or PostgreSQL directly.
+- Fast methods implementation of `Date` relative to `time.Time`.
+
+| Method        | package timex | package time |
+|:--------------|--------------:|-------------:|
+| OrdinalDate   |    4.07 ns/op |   6.28 ns/op |
+| Date          |    6.81 ns/op |   8.04 ns/op |
+| WeekDay       |    0.31 ns/op |   2.80 ns/op |
+| ISOWeek       |    7.52 ns/op |   9.80 ns/op |
+| Add           |   12.23 ns/op |  24.18 ns/op |
+| AddDays       |    0.31 ns/op |   3.12 ns/op |
+| Sub           |    0.31 ns/op |   6.53 ns/op |
+| Parse         |   34.72 ns/op |  55.47 ns/op |
+| Format        |   28.05 ns/op |  59.46 ns/op |
+| MarshalJSON   |   27.53 ns/op |  41.42 ns/op |
+| UnmarshalJSON |   10.89 ns/op |  52.11 ns/op |
+
+## Features
+
+- Fully-implemented type `Date`.
+    - Zero value: January 1, year 1. Align with type `time.Time`.
+    - Working with standard library: conversion with type `time.Time`.
+    - Parsing & Formatting: conversion with formatted strings.
+    - Getter: get year, quarter, month, day of year, day of month, day of week.
+    - Manipulation: addition and subtraction with years, month, days.
+    - Comparison: comparing dates with `Before`, `After`, `Equal`.
+    - Serialization: JSON and database.
+
+## Getting Started
+
+```
+go get github.com/invzhi/timex
+```
+
+## Reference
+
+- [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601)
+- [Date Type Proposal](https://github.com/golang/go/issues/21365)