UPDATE README.GIT-RULES wrt. PHP-7.3

We also add a respective note to README.RELEASE_PROCESS, so this won't
get overlooked again.

README.GIT-RULES

@@ -46,6 +46,10 @@ Currently we have the following branches in use::
 
   master    The active development branch.
 
+  PHP-7.3   Is used to release the PHP 7.3.x series. This is a current
+            non stable version and is open for bugfixes and minor improvements
+            only.
+
   PHP-7.2   Is used to release the PHP 7.2.x series. This is a current
             stable version and is open for bugfixes only.
 

README.RELEASE_PROCESS

@@ -341,6 +341,7 @@ Forking a new release branch
    Add a commit on master after the branch point clearing the NEWS, UPGRADING
    and UPGRADING.INTERNALS files, updating the version in configure.ac (run 
    ./configure to automatically update main/php_versions.h, too) and Zend/zend.h.
+   Also list the new branch in README.GIT-RULES.
      Example: http://git.php.net/?p=php-src.git;a=commit;h=a63c99b
    Push the new branch and the commit just added to master.