[Explorer] Update README with openssl and protoc dependencies

Summary: When building the explorer in a fresh new environment, cargo run will fail due to missing dependencies on openssl and protoc libraries. Have discussed this with Tobias who advised to add this to the README.

Test Plan:
use a fresh new dev environment
cd explorer-exe/
cp config.dist.toml config.toml
cargo run and observe the build failure
sudo apt install libssl-dev
sudo apt install -y protobuf-compiler libprotobuf-dev
cargo run
Go to http://localhost:3035 and ensure explorer loads locally

Reviewers: #bitcoin_abc, PiRK, bytesofman

Reviewed By: #bitcoin_abc, PiRK, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14900