Change markdown extension to .md

diff --git a/development.mdwn b/development.md
similarity index 100%
rename from development.mdwn
rename to development.md

diff --git a/index.mdwn b/index.md
similarity index 100%
rename from index.mdwn
rename to index.md

diff --git a/share/man/keyringer.1.mdwn b/share/man/keyringer.1.md
similarity index 100%
rename from share/man/keyringer.1.mdwn
rename to share/man/keyringer.1.md