SpaDES.docs 0.2.0

• new vignette, Building a project manual: the layout a manual uses, a runnable
  minimal example, what prepManualRmds() does to each module .Rmd and why,
  the build-script pattern, and the things that bite. Resolves the
  VignetteBuilder field that had been declared against no vignette (#2);
• the README says what the package is for, points at the vignette, and lists the
  manuals built with it. pkgdown builds the site home page from it, so it is also
  the front page of https://predictiveecology.github.io/SpaDES.docs/ (#3);
• Breaking: prepManualRmds() writes the generated chapters to a staging
  directory under the book root (stagingPath, default _manual_rmds) instead
  of into each module's own directory. Books must list the chapters from there
  in _bookdown.yml, and should add the directory to .gitignore. The module
  directories are git submodules in every project that uses this package: a
  failed build used to leave a <module>2.Rmd in each one, and each module
  repository carried a .gitignore line to hide it. Verified equivalent by
  rendering the same chapters from both locations -- the output is byte-identical,
  including relative images, cross-references and citations;
• chapters left by a previous run are cleared, so a module removed from a project
  no longer lingers as an orphan chapter;
• prepManualRmds() no longer fails when _bookdown.yml lists none of the
  modules under modulePath -- the case reported in #1, where the modules exist
  but every module line is commented out. It warns, writes the chapters, and
  skips the cross-chapter de-duplication it cannot do (#1);
• prepManualRmds() parses _bookdown.yml as YAML rather than by indentation.
  The old sub(" - ", ...) assumed exactly two spaces, could not read the
  flow-style rmd_files: [a, b] form, and counted a commented-out line as a
  listed chapter;
• prepManualRmds() gains argument bookdownYML, and checks the file exists
  before writing anything. A missing book file used to surface only after every
  <module>2.Rmd had been created, leaving them behind;