add readme note about optional build binaries
add readme note about optional build binaries

GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,tinytub/alertmanager,tyrken/alertmanager,Kellel/alertmanager,tyrken/alertmanager,prometheus/alertmanager,joehandzik/alertmanager,Kellel/alertmanager,tinytub/alertmanager,joehandzik/alertmanager,tinytub/alertmanager,GoogleCloudPlatform/alertmanager,Kellel/alertmanager,prometheus/alertmanager,Aloomaio/alertmanager,prometheus/alertmanager,joehandzik/alertmanager,tyrken/alertmanager,Aloomaio/alertmanager,GoogleCloudPlatform/alertmanager,tyrken/alertmanager,Aloomaio/alertmanager