Currently the instructions to install with hardsubx are vague and a new method was added in PR #966
This method makes installation with HARDSUBX easy and hence has been added to the documentation.
To build ccextractor with hardsubx support on linux, we need to configure
ccextractor with the `-enable-hardsubx` switch along with the
`ENABLE_HARDSUBX` flag passed during compilation with make. This commit
adds the missing configure instruction.