Updated documentation in README.md

several modules:

- _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.

  The main concepts used in TNL are the _memory space_, which represents the

  part of memory where given data is allocated, and the _execution model_,

  which represents the way how given (typically parallel) algorithm is executed.

  For example, data can be allocated in the main system memory, in the GPU

  memory, or using the CUDA Unified Memory which can be accessed from the host

  as well as from the GPU. On the other hand, algorithms can be executed using

  either the host CPU or an accelerator (GPU), and for each there are many ways

  to manage parallel execution. The usage of memory spaces is abstracted with

  [allocators][allocators] and the execution model is represented by

  [devices][devices]. See the [Core concepts][core concepts] page for details.

- _[Containers][containers]_.

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

  views, which abstract data management on different hardware architectures.

  views, which abstract data management and execution of common operations on

  different hardware architectures.

- _Linear algebra._

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

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

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

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

[allocators]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/namespaceTNL_1_1Allocators.html

[devices]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/namespaceTNL_1_1Devices.html

[core concepts]: https://mmg-gitlab.fjfi.cvut.cz/doc/tnl/core_concepts.html

[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