update changelog.md

Author: amfox37

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Updated read_obs_sm_ASCAT_EUMET to work with both original and revised file name templates. 
+- Updated subroutines read_obs_sm_ASCAT_EUMET(), read_obs_SMAP_halforbit_Tb(), read_obs_SMOS() and read_obs_MODIS_SCF() with hardcoded time ranges for when observations are available and should be read.
 
 ### Fixed