Update README with a more detailed usage section

[jneira]

Now it only suggests download the uberjar. It should mention:

• Include the library in a eta application via etlas, adding it int .cabalor etlas.dhall config file
• Include the library in a mixed project java/eta via eta gradle plugin, adding the dependency in build.gradle
• Comment about the caveats of the uberjar:
  • It includes classes from the bouncycastle java lib, so it can produce Class loading issues if bouncycastleis already in the classpath woth another version
  • Although not probable for now, it can clause clashes in eta internal classes if another version of eta is in the classpath