docs(contrib): Policy on manifest editing

Based on https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/Automatic.20edits.20to.20.60Cargo.2Ctoml.60

Author: epage

src/doc/contrib/src/SUMMARY.md

@@ -14,6 +14,7 @@
     - [New subcommands](./implementation/subcommands.md)
     - [Console Output](./implementation/console.md)
     - [Filesystem](./implementation/filesystem.md)
+    - [Formatting](./implementation/formatting.md)
     - [Debugging](./implementation/debugging.md)
 - [Tests](./tests/index.md)
     - [Running Tests](./tests/running.md)

src/doc/contrib/src/implementation/formatting.md

@@ -0,0 +1,17 @@
+# Formatting
+
+When modifying user files, like `Cargo.toml`, we should not change other
+sections of the file,
+preserving the general formatting.
+This includes the table, inline-table, or array that a field is being edited in.
+
+When adding new entries, they do not need to match the canonical style of the
+document but can use the default formatting.
+If the entry is already sorted, preserving the sort order is preferred.
+
+When removing entries,
+comments on the same line should be removed but comments on following lines
+should be preserved.
+
+Inconsistencies in style after making a change are left to the user and their
+preferred auto-formatter.