document Ubuntu limitations

[ThinkOpenly]

curt will not work on Ubuntu until support is added there in perf for the required Python bindings.

Document this in the README.

[arunsar]

let's explicitly mention that this tool can also work on x86, z and ppc.

[arunsar]

perf tools might not be installed on system, so add this as pre-req

[arunsar]

installing section needs to be changed to
git clone https://github.com/open-power-sdk/curt.git

[arunsar]

this info also needs to be added in readme

python scripting not supported. Install libpython and rebuild perf to enable it.
For example:

  # apt-get install python-dev (ubuntu)
  # yum install python-devel (Fedora)

etc.