📝 docs: fill out DEVELOPMENT.md (#129)

Author: JoshuaKGoldberg

.github/CONTRIBUTING.md

@@ -47,6 +47,7 @@ Be sure to fill out the pull request template's requested information -- otherwi
 PRs are also expected to have a title that adheres to [commitlint](https://github.com/conventional-changelog/commitlint) and uses a [gitmoji emoji](https://gitmoji.dev).
 Only PR titles need to be in that format, not individual commits.
 Don't worry if you get this wrong: you can always change the PR title after sending it.
+Check [previously merged PRs](https://github.com/LearningTypeScript/projects/pulls?q=is%3Apr+is%3Amerged+-label%3Adependencies+) for reference.
 
 Please try not to force-push commits to PRs that have already been reviewed.
 Doing so makes it harder to review the changes.

.github/DEVELOPMENT.md

@@ -1,8 +1,27 @@
 # Development
 
-TODO
+Thanks for your interest in developing on _Learning TypeScript_'s projects! 💖
 
-Will mention:
+> Be sure to read through [CONTRIBUTING.md](./CONTRIBUTING.md) and [CODE_OF_CONDUCT.md](./CODE_OF_CONDUCT.md) before contributing.
 
-- only one package.json
-- CI tasks as they're created
+## Project Structures
+
+Projects mostly have their directory and file structures standardized and verified by tests in `test/file.test.ts`.
+Each project should include a `tsconfig.json` in each "step" that extends the root `tsconfig.json` in this repository:
+
+- 🥗 Appetizers should have a TSConfig in their root and in each step
+- 🍲 Entrees and 🍰 desserts should have just the TSConfig in their root
+
+### Solutions
+
+Project solutions are stored as `solution.ts` and/or `*.solution.ts` files alongside starting files.
+Each project should have two scripts:
+
+- `test`: what learners run to verify their solutions have worked
+- `test:solution`: the same as `test`, but running on those `*solution.ts` files
+
+CI tasks verify project solutions:
+
+- [solutions.yml](./workflows/solutions.yml): that `test:solution` passes
+- [test.yml](./workflows/test.yml): that replacing original files with solutions passes `npm run test`
+- [tsc.yml](./workflows/tsc.yml): that replacing original files with solutions passes `npm run tsc`