add wilmersdorf theme

[ghost]

Description

• Adding a new theme: Wilmersdorf
• Updated README to include the steps that are listed in the pull request template & to mention how to get your screenshot to match the rest (this took me longer than I would like to admit until I found the PR that included the command from imagemagick)
• Update pull request template to mention resizing the screenshot

Theme Submission Checklist

• [X ] Included theme in iTerm2 format
• [X ] Included screenshot, 13pt Monaco font, no transparency
• [X ] Updated README.md with new theme and screenshot
• [X ] Updated CREDITS.md with new theme
• [X ] Ran tools/update_all.py to generate themes in all formats
• [ X] Updated screenshots/README.md with new theme

[mbadolato]

Thank you!