add simple README.md for all examples

examples/basic/README.md

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+### Description
+
+This is a very simplistic example where two waku nodes are instantiated within the same Rust app.
+
+### What it does
+
+1. Instantiates two Waku nodes
+2. Each node registers an event callback (waku message event, connection change event, etc.)
+3. Each node starts
+4. Each node perform relay subscription
+5. "node1" publishes a waku message
+6. Both nodes are stopped
+
+### How to run
+From within the `examples/basic/` foder run:
+```code
+cargo run
+```
+

examples/tic-tac-toe-gui/README.md

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+### Description
+
+This is a tic-tac-toe example that aims to show how to deal with
+a Waku node in an app with UI. The example is very naïve and it
+assumes only two tic-tac-toe instances are running globally. Therefore, the game messages might collide with other plays.
+
+The game board is shown within a Rust eframe.
+
+### What it does
+
+1. Instantiates one Waku node
+2. Starts the Waku node
+3. Registers the node to waku events (messages, connection change, topic health, etc.)
+4. Subscribes de node to the game_topic
+
+### How to run
+From within the `examples/tic-tac-toe/` foder run:
+```code
+cargo run
+```
+
+Another player can start his/her instance in either another
+terminal or another machine.
+

examples/toy-chat/README.md

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+### Description
+
+This is a chat example where multiple participants can talk within the same room.
+
+### What it does
+
+1. Instantiates one Waku node
+2. Starts the Waku node
+3. Registers the node to waku events (messages, connection change, topic health, etc.)
+4. Subscribes de node to the game_topic
+5. Retrieves previous chat messages at the beginning
+
+### How to run
+From within the `examples/toy-chat/` folder, run the following to start a chat using the given nick name.
+
+e.g.:
+```code
+cargo run "Alice"
+```
+
+