Document upgrade process from repmgr3

Also provide unpackaged extension upgrade SQL, and a script to
assist converting repmgr.conf files.

contrib/convert-config.pl

@@ -0,0 +1,88 @@
+#!/usr/bin/env perl
+
+use strict;
+use warnings;
+
+
+if (scalar @ARGV < 1) {
+    print qq|Please provide path to the repmgr.conf file as first parameter\n|;
+    exit(1);
+}
+
+my $file = $ARGV[0];
+
+if (!-e $file) {
+    print qq|Provide file "$file" does not exist\n|;
+    exit(1);
+}
+
+if (!-f $file) {
+    print qq|Provide path "$file" does not point to a file\n|;
+    exit(1);
+}
+
+my @outp = ();
+my $fh = undef;
+
+if (!open ($fh, $file)){
+    print qq|Unable to open "$file"\n|;
+    exit(1);
+}
+
+my $data_directory_found = 0;
+
+while(<$fh>) {
+    my $line = $_;
+    chomp $line;
+    if ($line =~ m|\s*#|) {
+        push @outp, $line;
+        continue;
+    }
+
+    if ($line !~ m|\s*(\S+?)\s*=(.+?)$|) {
+        push @outp, $line;
+        continue;
+    }
+
+    my $param = $1;
+    my $value = $2;
+
+    # These parameters can be removed:
+    next if ($param eq 'cluster');
+
+    # These parameters can be renamed:
+    if ($param eq 'node') {
+        push @outp, qq|node_id=${value}|;
+    }
+    elsif ($param eq 'failover') {
+        push @outp, qq|failover_mode=${value}|;
+    }
+    elsif ($param eq 'loglevel') {
+        push @outp, qq|log_level=${value}|;
+    }
+    elsif ($param eq 'logfacility') {
+        push @outp, qq|log_facility=${value}|;
+    }
+    elsif ($param eq 'logfile') {
+        push @outp, qq|log_file=${value}|;
+    }
+    elsif ($param eq 'master_reponse_timeout') {
+        push @outp, qq|async_query_timeout=${value}|;
+    }
+    else {
+        if ($param eq 'data_directory') {
+            $data_directory_found = 1;
+        }
+        push @outp, $line;
+    }
+}
+
+close $fh;
+
+print join("\n", @outp);
+print "\n";
+
+if ($data_directory_found == 0) {
+    print "data_directory=\n";
+}
+