blog: featured images (1200x630, ~90 KB) and a branding brief

Dotted simurg on a bead grid in the channel colors (red/green, gold bird,
indigo), one motif per post; generated by scripts/blog/make_featured_images.py.

blog/README.md

@@ -7,3 +7,7 @@ Five posts about why SimurgArray exists and where it is going.
 3. [What good could it do?](03_what_good_it_could_do.md)
 4. [Going public: what it takes, and a calendar](04_going_public_plan_and_calendar.md)
 5. [Using SimurgArray, and the bird behind the name](05_using_simurgarray_and_the_bird_behind_the_name.md)
+
+Featured images (1200 x 630 JPEG, about 90 KB each) are in `images/`, one per post with the same file name.
+They are drawn by `scripts/blog/make_featured_images.py` (Pillow), so they can be regenerated or restyled.
+`branding_prompt.md` is a brief for designing the full brand identity.