Add some instructions for the installation and use of Spglib's Python interface.

[hongyi-zhao]

Spglib's Python interface is used as an external dependency, so it should be first installed.

[goodluck1982]

I have add the dependence.

[hongyi-zhao]

In fact, your following description is not very accurate:

As you have written in the source code, the only one function that truly needs spglib is spglibGetSym:

$ rg -A5 autoConvTraceToBC::
SpaceGroupIrep.wl
247:autoConvTraceToBC::usage="autoConvTraceToBC[poscarFile,traceData,prec]  converts trace data to "<>
248-   "BC standard automatically according to POSCAR file and corresponding traceData. prec is optional "<>
249-   "and is 1*^-5 by default. This function calls spglibGetSym and hence needs external python package spglib. "<>
250-   "Note that the output is different if the option \"cellData\"->True is used, and in this case it is output[\"trace\"] "<>
251-   "that has the same data structure with traceData and should be used in getBandRep function.";
252-allBCSkLGdat::usage="allBCSkLGdat contains all little-groups ireps data of all space groups under BCS convention.";

[goodluck1982]

Althouth only spglibGetSym directly calls spglib, autoConvTraceToBC also needs spglib indirectly because it calls spglibGetSym.

[hongyi-zhao]

Agreed.