Doc request: archetypes for leaf bundles with Rmd in the book

[maelle]

After a bit of digging I found #263. Should the fact that you can create an index.Rmd archetype but that it has to be called index.md be documented in the book?

My current folder structure.

archetypes
├── Rmd
│   └── index.md
└── md
    └── index.md

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[yihui]

Archetypes are currently not documented in the blogdown book at all. To document the fact you mentioned, we need to document archetypes in the first place. Perhaps add a section in Chapter 2 after Section 2.3.