Make Git ignore Vim swapfiles.

Vim swapfiles typically end with `.swp`. In case of recovery, they may also end
with `.swo`.
Rewrite much documentation in markdown format.

The following files in the `doc` were rewritten in markdown format:

* DevelopersGuide.odt
* Proposal.odt
* Requirements.odt
* TopLevelDesign.odt
* UsersGuide.odt
Convert OpenDocument Drawing to dot format
Merge pull request #1 from Ichimonji10/master

update .gitignore, rewrite docs