Rectify the README with the latest details

- The readme document in the repo seems to have a small
  section which still refers to the older build system
  used by pldm.

- Now that, the repo is migrated to meson, this commit
  would attempt to rectify it.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: I470d477261758ca4443697812bca36e790c98641