Move markdown files and lexicon into .github directory (#205)

* Move markdown files and lexicon into .github directory
diff --git a/CONTRIBUTING.md b/.github/CONTRIBUTING.md
similarity index 100%
rename from CONTRIBUTING.md
rename to .github/CONTRIBUTING.md
diff --git a/SECURITY.md b/.github/SECURITY.md
similarity index 100%
rename from SECURITY.md
rename to .github/SECURITY.md
diff --git a/lexicon.txt b/.github/lexicon.txt
similarity index 100%
rename from lexicon.txt
rename to .github/lexicon.txt