docs: add TODO section comparing missing API with original DJB libtai

  Add comprehensive TODO section documenting API functions from the original
  DJB libtai implementation that are not yet implemented in this Go port:

  - TAIA (TAI64NA) high-precision time operations with attosecond precision
  - Calendar operations (caldate and caltime)
  - Advanced leap second management beyond current basic implementation

Signed-off-by: Nagy Károly Gábriel <[email protected]>

Author: karasz

README.md

@@ -255,6 +255,49 @@ make test                    # Full test suite
 
 [tai-wiki]: https://en.wikipedia.org/wiki/International_Atomic_Time
 
+## TODO
+
+The following API functions from the original DJB libtai implementation are not yet implemented in this Go port:
+
+### High-Precision Time (TAIA - TAI64NA)
+- **TAIA support**: TAI64NA (1-attosecond precision) time format and operations
+  - `taia_now()` - Get current time with attosecond precision
+  - `taia_add()`, `taia_sub()` - Arithmetic operations for TAIA
+  - `taia_pack()`, `taia_unpack()` - Serialization for 16-byte TAIA format
+  - `taia_less()` - Comparison operations
+  - `taia_half()` - Divide time by 2
+  - `taia_approx()` - Convert to floating-point approximation
+  - `taia_frac()` - Extract fractional part
+  - `taia_fmtfrac()` - Format fractional seconds
+
+### Calendar Date Operations (caldate)
+- **Calendar date handling**: Year-month-day operations and conversions
+  - `caldate_frommjd()` - Convert from Modified Julian Day number
+  - `caldate_mjd()` - Convert to Modified Julian Day number
+  - `caldate_normalize()` - Normalize invalid dates (e.g., Feb 30 → Mar 2)
+  - `caldate_fmt()`, `caldate_scan()` - String formatting and parsing
+  - `caldate_easter()` - Calculate Easter date for given year
+
+### Calendar Time Operations (caltime)
+- **Calendar time with timezone**: Complete date/time with UTC offset
+  - `caltime_tai()` - Convert from TAI to calendar time in UTC
+  - `caltime_utc()` - Convert from calendar time to TAI
+  - `caltime_fmt()`, `caltime_scan()` - String formatting and parsing
+
+### Leap Second Management (leapsecs)
+- **Advanced leap second handling**: Beyond the current basic implementation
+  - `leapsecs_init()` - Initialize leap second table
+  - `leapsecs_read()` - Read leap second data from file
+  - `leapsecs_add()` - Add leap seconds to TAI time
+  - `leapsecs_sub()` - Subtract leap seconds from TAI time
+
+### Missing Comparison and Utility Functions
+- **TAI64/TAI64N comparisons**: `tai_less()`, `tain_less()` for time ordering
+- **Validation functions**: Input validation for packed formats
+- **Extended arithmetic**: More comprehensive overflow handling
+
+**Priority**: Calendar operations (caldate/caltime) would provide the most value for general-purpose time handling, followed by TAIA support for ultra-high-precision applications.
+
 ## License
 
 This software is released into the public domain. See [UNLICENSE](UNLICENSE)