Update page 'Home'

Home.md

@@ -7,6 +7,7 @@
     * [The `append` subcommand](#the-append-subcommand)
 * [Clearing pedals](#clearing-pedals)
 * [Common problems](#common-problems)
+    * [Invalid combination of options!](#invalid-combination-of-options) 
 
 ---
 
@@ -102,5 +103,19 @@ OPTIONS:
 Thus, to clear pedals (e.g., 1 and 2), run: `footswitch-rs clear -p 1 2`.
 
 ## Common problems
-#### Invalid combination of options!
+### Invalid combination of options!
-This error can only occur with the `append` subcommand. 
+This error can only occur with the `append` subcommand. footswitch-rs can set the foot pedal into three different modes:
+
+```rust
+enum Type {
+    Unconfigured = 0,
+    Key = 1,
+    Mouse = 2,
+    MouseKey = 3,
+    String = 4,
+}
+```
+
+When appending something to a pedal, the application checks if this is valid. Obviously, this has not been the case when this error occurs. 
+
+This error often occurs when somenone tries to add a modifier (`footswitch-rs append modifier`) to one or more keys that were added with `footswitch-rs append key` instead of `footswitch-rs set key`. In the former case, the type of the key(s) is set to `Type::String`. In the latter case it is set to `Type::Key`. However, to append a modifier, `Type::Key` is required.