Add Totara 18 branch to readme. #591

Job	Run time
test / check if job needs to run	7s
test / prepare test matrix	17s
test / MOODLE_310_STABLE - pgsql - php 7.2 - tool_mfa	3m 29s
test / MOODLE_311_STABLE - mariadb - php 7.3 - tool_mfa	1m 44s
test / MOODLE_35_STABLE - mariadb - php 7.1 - tool_mfa	2m 23s
test / MOODLE_35_STABLE - mariadb - php 7.2 - tool_mfa	2m 3s
test / MOODLE_35_STABLE - pgsql - php 7.1 - tool_mfa	2m 13s
test / MOODLE_35_STABLE - pgsql - php 7.2 - tool_mfa	2m 21s
test / MOODLE_38_STABLE - mariadb - php 7.3 - tool_mfa	2m 32s
test / MOODLE_36_STABLE - mariadb - php 7.1 - tool_mfa	1m 58s
test / MOODLE_37_STABLE - pgsql - php 7.2 - tool_mfa	2m 13s
test / MOODLE_39_STABLE - mariadb - php 7.2 - tool_mfa	3m 12s
test / MOODLE_39_STABLE - mariadb - php 7.3 - tool_mfa	2m 58s
test / MOODLE_39_STABLE - pgsql - php 7.2 - tool_mfa	2m 36s
test / MOODLE_39_STABLE - pgsql - php 7.3 - tool_mfa	2m 39s
test / MOODLE_400_STABLE - pgsql - php 7.3 - tool_mfa	1m 54s
test / MOODLE_401_STABLE - pgsql - php 7.4 - tool_mfa	2m 21s
test / MOODLE_401_STABLE - mariadb - php 7.4 - tool_mfa	1m 50s
test / MOODLE_401_STABLE - pgsql - php 8.0 - tool_mfa	1m 58s
test / MOODLE_401_STABLE - mariadb - php 8.0 - tool_mfa	2m 5s
test / Release to the Moodle plugin directory	0s
	42m 53s

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