Add mainnet directory setup instructions.

[saniksin]

Description

Problem: The original instructions did not specify creating a dedicated directory for extracting snapshots. As a result, following the instructions led to Docker pointing to an incorrect path, causing synchronization to start from the first block instead of using the snapshot data.

Solution: Added commands to create a dedicated juno_mainnet directory and updated extraction instructions. This ensures the snapshot data is properly used during sync, avoiding unnecessary full blockchain synchronization from the start.

Changes Made:

1. Changes to directory creation command:

   - mkdir -p $HOME/snapshots
   + mkdir -p $HOME/snapshots/juno_mainnet

2. Changes to snapshot extraction command:

   - tar -xvf juno_mainnet.tar -C $HOME/snapshots
   + tar -xvf juno_mainnet.tar -C $HOME/snapshots/juno_mainnet

3. Example Docker run command after these changes:

docker run -d \
  --name juno \
  -p 6060:6060 \
  
  <!-- The problem is in the line: -->
  -v $HOME/snapshots/juno_mainnet:/snapshots/juno_mainnet \
  
  nethermind/juno \
  --http \
  --http-port 6060 \
  --http-host 0.0.0.0 \
  --db-path /snapshots/juno_mainnet \
  --eth-node <YOUR ETH NODE>

These changes ensure that the Docker container will use the correct path, utilizing the snapshot effectively instead of resynchronizing from the genesis block.