README

[ciasom]

Hi Javier,

Thanks for the great tutorial. Actually I didn't get any idea of how to write an extension after reading https://spark.rstudio.com/extensions/.

I'd like some changes for the README files.

Firstly, the following point is not accurate.
2. You should download and install the Scala 2.10 and 2.11 compilers to one of the following paths:
- /opt/scala
- /opt/local/scala
- /usr/local/scala
- ~/scala (Windows-only)

Scala doesn't need to be in the above locations. I built a jar on Ubuntu with scala 2.10 and 2.11 in my home directory like the following:
scalas/
├── scala-2.10.7
└── scala-2.11.8

And by using compilation specification to specify the location of scala, the compile_package_jars function was able to successfully build the jar.

spec <- spark_default_compilation_spec(locations = "/home/ciasom/scalas")
compile_package_jars(spec=spec)

Secondly, it would be better if examples of how the jars are used in other situations other than local spark, for instance, through Livy, can be added.

[wendywangwwt]

I'm confused by the gap between having the r functions and loading sparkhello as a library. I read through the readme and couldn't understand how to tell R that this sparkhello is a library, if I need to install it and how (we only generated jar files I think?), and how to use the two R functions. They are possibly related or unrelated 😅 it would be great if someone could answer my questions and/or put them into the readme.