Add release notes

RELEASE-NOTES.txt

@@ -1,5 +1,39 @@
 Stan Math Library Release Notes
 
+======================================================================
+v5.0.0 (10 December 2024)
+======================================================================
+
+Breaking changes:
+
+ - Removed deprecated `_log()` probability functions. (#2995)
+ - Moved constraint transforms to their own folder. (#3039)
+ - Updated arena_matrix to use move semantics. (#2928)
+ - Un-vendored a great many unused boost libraries. (#3129)
+ - C++17 is now required.
+
+ Other changes:
+
+ - Added `sum_to_zero_constrain`, `sum_to_zero_free`, and `check_sum_to_zero`. (#3101)
+ - Added beta_neg_binomial_lpmf. Allows modeling heavy-tail count data. (#3108)
+ - Added beta_neg_binomial_lccdf. (#3114, #3118)
+ - Added beta_neg_binomial_lcdf. (#3116)
+ - Added beta_neg_binomial_cdf. (#3120)
+ - Added beta_neg_binomial_rng. (#3126)
+ - Added overloads for tuples of bounds to `lub_free` and `lub_constrain`. (#3087)
+ - Added error checking functions for row/column stochastic matrices. (#3095)
+ - Added constructor to vari for passing initial values and adjoints. (#3089)
+ - Improved efficiency of `finite_diff_gradient_auto`. (#3096)
+ - Functions concerned with the number of entries in a container (`size`, `cols`, `rows`, ...) now consistently use `long int` as their return type. (#3086)
+ - Removed support for adding makefile variables in `~/.config/stan/make.local`. Use the `make/local` file in the Math directory instead. (#3090)
+ - Fixed an indexing bug inside gp_matern32_cov. (#3084)
+ - Fixed the new `wiener_lpdf` overloads evaluating their expression template arguments more than once. (#3112)
+ - Fixed some failures to compile calls to `pow` when using libc++ 19. (#3110)
+ - Updated templated complex functions in the Stan math library to be better candidates than the standard library functions during ADL. (#3122)
+ - Fixed a warning about a mismatched struct/class declaration from clang 18. (#3091)
+ - Fixed compilation of `fabs` with Apple Clang 15. (#3093)
+ - Fix Eigen issue [2852](https://gitlab.com/libeigen/eigen/-/issues/2852) so that Eigen's kronecker product type works  with `value_type_t`. (#3124)
+
 ======================================================================
 v4.9.0 (3 June 2024)
 ======================================================================