Add hacktoberfest-related documentation (#2616)

* git add HACKTOBERFEST.md

* clarify contributions

* respond to review comments

* add link to HACKTOBERFEST.md from README.md

* typo

* include comments from Gordon

HACKTOBERFEST.md

@@ -0,0 +1,35 @@
+# :pizza: Hacktoberfest 2019 :beer:
+
+It's that time of the year again!
+[Hacktoberfest](https://hacktoberfest.digitalocean.com) is here, and `gensim` needs **your** help.
+We've prepared a list of good issues to work on: [gensim hacktoberfest issues](https://github.com/RaRe-Technologies/gensim/labels/hacktoberfest).
+
+If the learning curve for `gensim` is a bit steep, give the [smart_open](https://github.com/RaRe-Technologies/smart_open) repository a try.
+`smart_open` is an important dependency of `gensim`: it performs file I/O over a variety of protocols and formats.
+There's also a list of Hacktoberfest-friendly issues to work on: [smart_open hacktoberfest issues](https://github.com/RaRe-Technologies/smart_open/labels/hacktoberfest).
+
+Of course, we welcome contributions on any of the existing issues, not just the ones labeled `hacktoberfest`.
+If the issue is simple & quick, you can just submit your PR, with a proper reference to the issue it addresses.
+If the issue requires a little more work, but you have a good idea of how to proceed & know when you'll be submitting some initial work, please post a short note about your plans to the issue, or a "work-in-progress" ("[WIP]") pull-request indicating work is underway, to help avoid wasted duplicate work.
+
+Furthermore, we also welcome contributions not connected to an existing issue.
+This includes things like fixing typos in documentation, docstrings, etc.
+If you make such contributions, please make the motivation behind the contribution clear.
+You could start such a contribution with a new pull-request, or if you think it requires other discussion beforehand, as a separate new issue.
+Please avoid making innocuous changes without sufficient motivation (e.g. changing code formatting, etc).
+
+## Before Contributing
+
+Check out the following:
+
+- [First-time contributors guide](https://github.com/firstcontributions/first-contributions): if this is your first time contributing on GitHub.
+- [Hacktoberfest rules](https://hacktoberfest.digitalocean.com/faq#rules): read this in full
+- [Developer page](https://github.com/RaRe-Technologies/gensim/wiki/Developer-page) on our Wiki: for the git flow, code style, etc.
+
+## Questions
+
+If you have a general question about Gensim, please ask on the [mailing list](https://groups.google.com/forum/#!forum/gensim).
+If you have a question a about a specific issue or PR, just ask there directly, and we'll get back to you as soon as we can.
+Otherwise, ping @mpenkov on [Twitter](https://twitter.com/mpenkov) or [Telegram](https://t.me/mpenkov).
+
+Happy Hacking!!

README.md

@@ -10,13 +10,16 @@ gensim – Topic Modelling in Python
 [![Gitter](https://img.shields.io/badge/gitter-join%20chat%20%E2%86%92-09a3d5.svg)](https://gitter.im/RaRe-Technologies/gensim)
 [![Follow](https://img.shields.io/twitter/follow/gensim_py.svg?style=social&label=Follow)](https://twitter.com/gensim_py)
 
-
-
 Gensim is a Python library for *topic modelling*, *document indexing*
 and *similarity retrieval* with large corpora. Target audience is the
 *natural language processing* (NLP) and *information retrieval* (IR)
 community.
 
+## :pizza: Hacktoberfest 2019 :beer:
+
+We are accepting PRs for Hacktoberfest!
+See [here](HACKTOBERFEST.md) for details.
+
 Features
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