add README for logogen

src/logogen/README.md

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+Logo generation
+=================
+
+## Logo properties
+* adds a wormhole logo at the bottom right
+* if symbol has multiple source chains (e.g. USDC), adds a chain logo at the bottom left
+
+Examples:
+
+CAKE:
+
+![CAKE](../../assets/CAKE_wh.png)
+
+USDTpo:
+
+![USDTpo](../../assets/USDTpo_wh.png)
+
+## Instructions
+
+### Base logos
+Base logos should be supplied either by adding png files to [src/logogen/base/](base/),
+or by adding a logo component to [src/config/token_data.py](../config/token_data.py).
+If both are present, we'll use the one in [src/logogen/base/](base/),
+
+### Generating new logos
+```
+npm run genlogos
+```
+
+### Linking to logos
+```
+https://raw.githubusercontent.com/certusone/wormhole-token-list/main/assets/CAKE_wh.png
+```