Added preliminary parser description to the docs

This is a description of the preliminary design for the parser of
PROJCAR/LOCPROJ files output by VASP.

doc/vasp/source/vaspio.rst

@@ -3,7 +3,27 @@
 VASP input-output
 #################
 
-The following VASP are used by PLOtools:
+The following VASP files are used by PLOtools:
-  * PLOCAR: raw projectors generated by VASP-PLO interface
+  * PROJCAR, LOCPROJ: raw projectors generated by VASP-PLO interface (VASP version >= 5.4.1)
-  * EIGENVAL: Kohn-Sham eigenvalues as well as `k`-points with weights
+  * EIGENVAL: Kohn-Sham eigenvalues as well as `k`-points with weights and Fermi weights
   * IBZKPT: `k`-point data (:math:`\Gamma`)
+  * POSCAR: crystal structure data
+
+Starting from version 5.4.1 VASP now supports an official output of various types of
+projectors that are requested in INCAR by specifying a set of sites, orbitals and types
+of projectors. The calculated projectors are output into files **PROJCAR** and **LOCPROJ**.
+The difference between these two files is that **LOCPROJ** contains raw matrices without
+any reference to sites/orbitals, while **PROJCAR** is more detailed on that.
+In particular, the information that can be obtained for each projector from **PROJCAR** is the following:
+
+  * site (and species) index
+  * for each `k`-point and band: a set of complex numbers for labeled orbitals
+
+At the same time, **LOCPROJ** contains the total number of projectors (as well as the
+number of `k`-points, bands, and spin channels) in the first line,
+which can be used to allocate the arrays before parsing.
+
+To enhance the performance of the parser, it is implemented in plain C. The idea is
+that the python part of the parser first reads the first line of **LOCPROJ** and
+then calls the C-routine with necessary parameters to parse **PROJCAR**.
+