include LICENSE file in published crates

[decathorpe]

The MIT License terms require that the license text is made available with any redistributed sources, which also means that the LICENSE file should be included in crates published to crates.io (capnp, capnpc, capnp-futures, capnp-rpc).

To have "cargo publish" pick the files up, adding symlinks LICENSE → ../LICENSE in each of the workspace member directories (at least those that are actually published) is enough.

[dwrensha]

Good point! Done in 24938b7.

[decathorpe]

Thank you!