docs: schema: add tasks.task.method

[derhuerst]

As documented in

task/website/src/docs/guide.md

Lines 816 to 830 in 90bcbe9

	At the task level for a specific task:
	```yaml
	version: '3'
	tasks:
	build:
	cmds:
	- go build .
	sources:
	- ./*.go
	generates:
	- app{{exeExt}}
	method: timestamp
	```

.

[vmaerten]

Thanks !