Document environment variables in manual

manual/src/usage.md

@@ -31,3 +31,15 @@ extension, and ignore other language detection heuristics.
 ```
 $ difft --language cpp before.c after.c
 ```
+
+## Options
+
+Difftastic includes a range of configuration CLI options, see `difft
+--help` for the full list.
+
+Difftastic can also be configured with environment variables. These
+are also visible in `--help`.
+
+For example, `DFT_BACKGROUND=light` is equivalent to
+`--background=light`. This is useful when using VCS tools like git,
+where you are not invoking the `difft` binary directly.