readme: Add information on replacement of the library in go.mod

[Bidon15]

If someone wants to use celestia-app as a dependency in their project, then it will be handy to include in the readme that this specific replacement liner should be included in the project's go.mod

github.com/gogo/protobuf => github.com/regen-network/protobuf v1.3.3-alpha.regen.1

[Bidon15]

Actually, it's better to include the whole replace part

replace (
    github.com/99designs/keyring => github.com/cosmos/keyring v1.1.7-0.20210622111912-ef00f8ac3d76
    github.com/cosmos/cosmos-sdk => github.com/celestiaorg/cosmos-sdk v1.0.1-sdk-v0.44.2
    github.com/gogo/protobuf => github.com/regen-network/protobuf v1.3.3-alpha.regen.1
    github.com/tendermint/tendermint => github.com/celestiaorg/celestia-core v1.0.1-tm-v0.34.16
    google.golang.org/grpc => google.golang.org/grpc v1.33.2
)