Document the existence of `TRITON_HOME` environment variable.

The `TRITON_HOME` variable controls the location of the `.triton`
directory that stores, among other things, the files downloaded during a
`pip install -e python` virtualenv build. By default, this is located in
the user's home directory, at `~/.triton`.

I was trying to build Triton on my system on a large local disk, but
with limited network home directory space, and the `pip` command kept
failing with out of disk space errors. It turned out that during
installation, large files were downloaded to the `~/.triton` directory
causing failure.

After checking that it was not `pip` doing this, I found the
`TRITON_HOME` variable which allowed me to workaround the issue and
build Triton successfully. After seconding #4007, I decided to
contribute this documentation fix.

Co-authored-by: sree <sree@buckyball>