Documentation: Main Readme refers to parameters that don't exist

The Readme on the front page makes this reference:

"How can I parameterize my Rocket chip?

By now, you probably figured out that all generated files have a configuration name attached, e.g. freechips.rocketchip.system.DefaultConfig. Take a look at src/main/scala/system/Configs.scala. Search for NSets and NWays defined in BaseConfig. You can change those numbers to get a Rocket core with different cache parameters. For example, by changing L1I, NWays to 4, you will get a 32KB 4-way set-associative L1 instruction cache rather than a 16KB 2-way set-associative L1 instruction cache. Towards the end, you can also find that DefaultSmallConfig inherits all parameters from BaseConfig but overrides the same parameters of WithNSmallCores."

However, the file referred to by the Readme makes no mention of cache ways and sets. That is in `subsystem` where the cores are defined.

Start with the front-page README passage and compare its reference to src/main/scala/system/Configs.scala and the subsystem code where the cores and cache parameters are defined. Update the documentation so its file path and parameter references match the current code, then verify that the configuration examples describe existing parameters.

Written by the indexing model from the issue text.