docs: Revert the moving of design.md

This commit fix the issue reported in #803:
https://github.com/JuulLabs-OSS/mcuboot/issues/803

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
diff --git a/scripts/imgtool/design.md b/docs/design.md
similarity index 100%
rename from scripts/imgtool/design.md
rename to docs/design.md