add todo

Merge pull request #51 from IgorPolyakov/master

fix typo

update license links

Merge pull request #49 from IgorPolyakov/master

Fixed some typos in `extensions` part

HTML generation

Merge pull request #46 from olshevskiy87/t12

fix typos in strategy chapter

Merge pull request #44 from olshevskiy87/t10

fix typos in indexes chapter

Merge pull request #42 from olshevskiy87/t9

fix backup and clustering punctuation an typos

Merge pull request #39 from olshevskiy87/t6_clustering_2

fix typos in clustering chapter, postgres_xl

Merge pull request #37 from olshevskiy87/t5_cache

fix cache chapter