README.md: added links to the documentation

Similarly to the STL, features provided by the TNL can be grouped into

several modules:

> TODO: link relevant terms to the Doxygen documentation

- _Core concepts_.

  The main concept used in the TNL is the `Device` type which is used in most of

  the other parts of the library. For data structures such as `Array` it

  specifies where the data should be allocated, whereas for algorithms such as

  `ParallelFor` it specifies how the algorithm should be executed.

- _Containers_.

- _[Containers][containers]_.

  TNL provides generic containers such as array, multidimensional array or array

  views, which abstract data management on different hardware architectures.

- _Linear algebra._

  TNL provides generic data structures and algorithms for linear algebra, such

  as vectors, sparse matrices, Krylov solvers and preconditioners.

  as [vectors][vectors], [sparse matrices][matrices],

  [Krylov solvers][linear solvers] and [preconditioners][preconditioners].

   - Sparse matrix formats: CSR, Ellpack, Sliced Ellpack, tridiagonal,

     multidiagonal

   - Krylov solvers: CG, BiCGstab, GMRES, CWYGMRES, TFQMR

   - preconditioners: Jacobi, ILU(0) (CPU only), ILUT (CPU only)

- _Meshes_.

   - Preconditioners: Jacobi, ILU(0) (CPU only), ILUT (CPU only)

- _[Meshes][meshes]_.

  TNL provides data structures for the representation of structured or

  unstructured numerical meshes.

- _Solvers for differential equations._

  TNL provides a framework for the development of ODE or PDE solvers.

- _Image processing_.

- _[Image processing][image processing]_.

  TNL provides structures for the representation of image data. Imports and

  exports from several file formats are provided using external libraries, such

  as [DCMTK](http://dicom.offis.de/dcmtk.php.en) for DICOM files,

  [libpng](http://www.libpng.org/pub/png/libpng.html) for PNG files, or

  [libjpeg](http://libjpeg.sourceforge.net/) for JPEG files.

For more information, see the [full documentation](#documentation).

[containers]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/namespaceTNL_1_1Containers.html

[vectors]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/classTNL_1_1Containers_1_1Vector.html

[matrices]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/namespaceTNL_1_1Matrices.html

[linear solvers]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/namespaceTNL_1_1Solvers_1_1Linear.html

[preconditioners]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/namespaceTNL_1_1Solvers_1_1Linear_1_1Preconditioners.html

[meshes]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/namespaceTNL_1_1Meshes.html

[image processing]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/namespaceTNL_1_1Images.html

For more information, see the [full documentation][full documentation].

## Installation

## Documentation

> TODO: link to the Doxygen documentation

See the [full documentation][full documentation] for the API reference manual,

tutorials and other documented topics.

[full documentation]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/

## Authors