Update README

README.md

@@ -19,6 +19,7 @@ usage: gersemi [-c] [-i] [--diff] [--default-config] [--version] [-h] [-l INTEGE
                [--indent (INTEGER | tabs)] [--unsafe] [-q] [--color]
                [--definitions src [src ...]]
                [--list-expansion {favour-inlining,favour-expansion}] [-w INTEGER]
+               [--cache] [--require-definitions]
                [src ...]
 
 A formatter to make your CMake code the real treasure.
@@ -71,6 +72,16 @@ configuration:
   -w INTEGER, --workers INTEGER
                         Number of workers used to format multiple files in parallel.
                         [default: number of CPUs on this system]
+  --cache, --no-cache   Enables cache with data about files that are known to be
+                        formatted to speed up execution.
+                        [default: cache enabled]
+  --require-definitions, --no-require-definitions
+                        Require definitions of custom commands. When enabled file which
+                        has unknown custom commands will have warnings issued about
+                        that and result won't be cached. When disabled it will be
+                        assumed that original formatting of unknown command is the
+                        correct one. See: "Let's make a deal" section in README.
+                        [default: definitions are required]
 ```
 
 ### [pre-commit](https://pre-commit.com/) hook
@@ -442,6 +453,8 @@ watch_david_fincher_movies(
 ```
 If you find these limitations too strict let me know about your case.
 
+When source code has custom commands but their definitions aren't known `gersemi` will warn about that. Warnings can be suppressed with `--no-required-definitions`/`required_definitions: false`.
+
 #### How to format custom commands for which path to definition can't be guaranteed to be stable? (e.g external dependencies not managed by CMake)
 
 You can provide stub definitions that will be used only as an input for gersemi. Example: