Add a principles and guidelines section

Includes discussion of trailing commas which closes #42

guide/guide.md

@@ -100,6 +100,8 @@ struct CRepr {
 }
 ```
 
+## [Guiding principles and rationale](principles.md)
+
 ## [Non-formatting conventions](advice.md)
 
 ## [Cargo.toml conventions](cargo.md)

guide/principles.md

@@ -0,0 +1,51 @@
+# Guiding principles and rationale
+
+When deciding on style guidelines, the style team tried to be guided by the
+following principles (in rough priority order):
+
+* readability
+    - scan-ability
+    - avoiding misleading formatting
+    - accessibility - readable and editable by users using the the widest
+      variety of hardware, including non-visual accessibility interfaces
+    - readability of code when quoted in rustc error messages
+
+* aesthetics
+    - sense of 'beauty'
+    - consistent with other languages/tools
+
+* specifics
+    - compatibility with version control practices - preserving diffs,
+      merge-friendliness, etc.
+    - preventing right-ward drift
+    - minimising vertical space
+
+* application
+    - ease of manual application
+    - ease of implementation (in Rustfmt, and in other tools/editors/code generators)
+    - internal consistency
+    - simplicity of formatting rules
+
+
+## Overarching guidelines
+
+Prefer block indent over visual indent. E.g.,
+
+```
+// Block indent
+a_function_call(
+    foo,
+    bar,
+);
+
+// Visual indent
+a_function_call(foo,
+                bar);
+```
+
+This makes for smaller diffs (e.g., if `a_function_call` is renamed in the above
+example) and less rightward drift.
+
+Lists should have a trailing comma when followed by a newline, see the block
+indent example above. This choice makes moving code (e.g., by copy and paste)
+easier and makes smaller diffs.