More stable PEL with several new features and documentation

Although I don't manage PEL through a rigorous development process, and I still consider PEL in its early development cycle, I create release tags at some points to identify its evolution.

• There were lots of updates since version 0.1.0, too many to describe here.
  PEL is still in what I consider an early development phase.
  Although most base features are stable, there's still several rough edges,
  lots of packages and programming languages I want to support are not
  properly supported.

  • I changed the instructions for installations on a new computer.
  • Added lots of cross-reference links between the PDF sheets.
  • Implemented a tempo skeleton generic file header block generation, and
    specialized ones for C, Erlang and reStructuredText files.
  • Support for Erlang and C is mostly complete, but there are several packages
    not supported yet.
  • Added support for sessions, projectile, hiding comments, hiding docstrings.
  • Added ability to automatically download and install packages from some
    non-Elpa compliant web sites (but path of Github) like EmacsAttic and
    EmacsMirror.
  • Support for generation of images using PlantUML and Graphviz Dot directly
    from within Emacs.
  • Support for several ways to deals with keys: Hydras, key-chord and key-seq,
    avy and ace-link.
  • Support for voice dictation under macOS and AppleScript.
  • Support for auto-completion using either auto-complete or company.
  • Support for input completion with Help, Ivy, Ivy/Counsel as well as Ido.
  • Support for bookmark management with bm.
  • Support for virtual-regexp
  • Key bindings to quickly access the customization buffer of several
    customization groups.
  • and more...