Merge branch 'master' into dev

CONTRIBUTING.md

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+Hi,
+
+first of all, thank you you are considering to contribute to sumy. Because people started to send
+more Pull request I would like to set some rules here to make it easier and faster to review PR
+with your code. Nothing is written in stone but please try to respect them.
+
+1. If the change is not few lines of code **ask if the feature is needed**. I would like to avoid
+   situation when **week of work will be rejected** because the feature is something what should 
+   be in sumy. On the other hand if it's **10 lines it's easier to send PR** with changes and discuss 
+   the code in PR. If it would be rejected it's 10 still lines of code so not a big deal (I hope) :)
+2. If you add some new feature please **add tests** for it. Also if you fix a bug write test for it.
+   I don' believe fixes without tests. If you don't know how **ask me for help** in PR with code.
+3. When the PR changes something what should be known to end user please **add a line into CHANGELOG.md**.
+4. Please **respect the code style**. I don't want to set specific strict rules here because I think it's 
+   possible to respect style from all the code around. For example you will not use `'` for strings 
+   when you see that the code uses `"` ...
+
+And that's all. I am looking forward for your PR :)