Update changelog

Closes #1855

Author: jeromekelleher

CHANGELOG.md

@@ -1,7 +1,12 @@
 # Changelog
 
 
-## [1.3.5] - 2025-XX-XX
+## [1.4.0] - 2025-XX-XX
+
+**New features**
+
+- The `FixedPedigree` simulation model now supports internal samples ({issue}`1855`,
+  {pr}`2321`, {pr}`2326`, {pr}`2388`, {user}`abureau`, {user}`jeromekelleher`).
 
 **Breaking changes**:
 
@@ -33,7 +38,7 @@ Bugfix release for issues with Dirac and Beta coalescent models.
 
 - Add `record_provenance` argument to `sim_mutations` ({issue}`2272`, {pr}`2273`, {user}`peterlharp`)
 
-- Add support and wheels for Python3.12, drop Python3.8 
+- Add support and wheels for Python3.12, drop Python3.8
 
 ## [1.3.1] - 2024-02-09
 
@@ -48,7 +53,7 @@ Bugfix release for issues with Dirac and Beta coalescent models.
 - Add a `MicrosatMutationModel` mutation model class, that
   represents a generalized interface for constructing mutational
   models appropriate to STRs. In addition 3 specific microsat models
-  are added `SMM`, `TPM`, and `EL2`.  
+  are added `SMM`, `TPM`, and `EL2`.
   ({issue}`2013`, {user}`andrewkern`).
 
 - Raise an error if `log_arg_likelihood` is called on a recombinant tree