[🔥AUDIT🔥] Add white background for social preview image (#122)

🖍 _This is an audit!_ 🖍

## Summary:
This has the same dark mode problem (if your Twitter client, say, is in
dark mode), but we can't be so fancy with CSS, I uess.  So I just
switched to a white background, which looks fine in context.  The actual
source of truth for the social preview image is in the github settings
page, but I checked in the image for reference.

## Test plan:
looks fine in the compose window on Facebook:
![Screenshot 2021-09-30 at 09-34-49 Facebook](https://user-images.githubusercontent.com/111238/135495585-39208971-04d8-4c56-9df1-61970504858b.png)

Author: benjaminjkraft

Auditors: dnerdy

Required Reviewers: 

Approved By: 

Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ✅ Lint, ⌛ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: #122

Author: benjaminjkraft

docs/images/README.md

@@ -1,5 +1,11 @@
 # genqlient logo
 
-`genqlient-orig.svg` is the original logo.  To create `genqlient.svg`, I:
+`genqlient-orig.svg` is the original logo, in editable SVG.  To create `genqlient.svg`, I:
 - converted all text to paths
 - added media queries to make the paths white if the user is in dark mode (see issue #17)
+
+For the social preview image (in GitHub settings), I ran:
+```sh
+convert -resize 2560x1280 -density 600 -gravity center \
+  -extent 2560x1280 docs/images/genqlient.{svg,png}
+```