Update FEATURES.md with HiPO release notes

Update of FEATURES.md

Author: jajhall

FEATURES.md

@@ -1,3 +1,13 @@
 ## Code changes
 
+The HiPO release exposed various issues flagged up via GitHub and email.
+- Fix some overflows when computing statistics of analyse phase.
+- Free memory used for normal equations, if augmented system is preferred.
+- Fix bug in supernode amalgamation.
+- Printing of BLAS library moved to HiGHS header, so it is printed when using HiPO without logging.
+- Recommend to install Metis branch `521-ts` due to better oredring quality on many problems. Update workflows and documentation accordingly.
+- Add option `hipo_metis_no2hop` to control option `no2hop` of Metis and add warning if the fill-in is large.
+
+Added singleton column stuffing to MIP presolve - see Gamrath et al., Progress in presolving for mixed integer programming. Math. Prog. Comp. 7, 367–398 (2015).
+
 ## Build changes