docs: add style guide

[skip ci]

STYLE.md

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+# Alejandra's Style Guide
+
+Alejandra's mission is to produce a **consistent style**
+that is **easy to read**
+and produces **clean diffs**.
+This means trading aggressively compact code
+for regularity and ease of modification.
+
+## If-Then-Else
+
+✅ Good:
+
+```nix
+if predicate
+then foo
+else bar
+```
+
+- The keyword at the beginning of the line
+  states clearly the meaning of the content that follows.
+- Produces a clean diff when you add more code.
+  For example: adding content to the `else`
+  only produces a diff in the `else`.
+
+❌ Bad:
+
+<!-- nixfmt -->
+
+```nix
+if predicate then foo else bar
+```
+
+- One-liners are hard to understand,
+  specially when nested,
+  or when logic gets long.
+- Adding content produces a diff in the entire `if-then-else`.
+
+✅ Good:
+
+```nix
+if something <= 2.0
+then
+  if somethingElse
+  then foo
+  else bar
+else if something <= 4.0
+then baz
+else if something <= 6.0
+then foo
+else bar
+```
+
+- It's easy to follow that there are many conditionals.
+- The indentation makes it easy to read
+  which expression is associated to each conditional.
+- Adding or modifying the branches produces a clean diff.
+
+❌ Bad:
+
+<!-- nixpkgs-fmt -->
+
+```nix
+  if cond
+  then if
+    looooooooooooooooooooooooooooooooooooooooooooooooooooong
+  then foo
+  else bar
+  else if cond
+  then foo
+  else bar
+```
+
+- It's complex to distinct the parent `if-then-else`
+  from the child `if-then-else`