doc: add procedure

doc/procedure.md

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+# Procedure
+1. Open `TestTestExe`
+2. Go to `Test > Options`:
+    1. Tab `Auto`: Stop test after Trial **13**, Switch to test mode after trial **3**
+    2. Tab `Prevention`: Prevent **Punctuation, numbers, capitals**
+3. determine keyboard testing sequence
+4. ask participant age
+5. Repeat for each keyboard:
+    1. Set keyboard
+    2. Participant fulfills typing task
+    3. Note down `wpm` and `ter` in results.csv
+6. Present reduced NASA-TLX