Update readme with year over year instructions

README.md

@@ -23,6 +23,31 @@ go install github.com/nmoniz/any2anexoj/cmd/any2anexoj-cli@latest
 cat statement.csv | any2anexoj-cli --platform=tranding212
 ```
 
+## Incremental Reports
+
+For multi-year reporting, pass `--state-file <path>` to keep the FIFO buy-queue between runs. The typical workflow is:
+
+1. Generate the first year Anexo J report from the first year broker statement and keep the state file.
+2. Next year, generate the next report from the second year broker statement **only**, using the same state file.
+
+```bash
+# first year
+cat 2024-statement.csv | any2anexoj-cli --platform=trading212 --state-file=trading212-state.json
+
+# following year
+cat 2025-statement.csv | any2anexoj-cli --platform=trading212 --state-file=trading212-state.json
+```
+
+The first run behaves like a normal run and creates the state file.
+On later runs the tool loads the previously saved buy queues per symbol (including partially filled lots), processes only the new transactions from the input, and writes the updated state.
+The state file is a JSON document.
+
+> [!IMPORTANT]
+> Each new input must contain **only** the new transactions, not the previous ones.
+> The state file already holds the unmatched buy lots from earlier runs, so including previous trades again will cause them to be processed twice and may fail with insufficient bought volume.
+
+State is only persisted once the input reaches EOF, so a crash or `Ctrl-C` before the end of input will discard progress from that run.
+
 ## Rounding
 
 All Euro values are rounded to cents (2 decimal places) but internal calculations use the statement values with full precision.