Add doc for pmgen semioptional statement, Add pmgen changes to CHANGELOG

Signed-off-by: Clifford Wolf <clifford@clifford.at>

CHANGELOG

@@ -19,6 +19,7 @@ Yosys 0.9 .. Yosys 0.9-dev
     - Added automatic gzip compression (based on filename extension) for backends
     - Improve attribute and parameter encoding in JSON to avoid ambiguities between
       bit vectors and strings containing [01xz]*
+    - Improvements in pmgen: subpattern and recursive matches
 
 Yosys 0.8 .. Yosys 0.8-dev
 --------------------------

passes/pmgen/README.md

@@ -175,6 +175,9 @@ explore the case where `mul` is set to `nullptr`. Without the `optional`
 statement a match may only be assigned nullptr when one of the `if` expressions
 evaluates to `false`.
 
+The `semioptional` statement marks matches that must match if at least one
+matching cell exists, but if no matching cell exists it is set to `nullptr`.
+
 Additional code
 ---------------