Extend instructions for writing blog posts

- Mention the caveat about posts with a date in the future

- Introduce the `_drafts` folder to be used for collaborative posts.

Note: The `_drafts` folder doesn't exist yet; I intend to create it
in a following commit.

Change-Id: Ifa936840200bfbe2240ad2410f4b6f90c418dd6d
1 file changed