Added Graph Gallery, updated website

• Added a Graph Gallery to README.md
• Added About section to README.md
• Updated About page on site
• Fixed {purrr} misspelling
• Added table of contents to README.md
• Removed "Welcome" post from site

Initial release of #TidyTuesday blog

• First 15 blog posts are now available, with full R Markdown (.Rmd) and R (.R) source files