Add explanations of encryption methods

docs/encryption.md

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+Dear users,
+
+Pix-Art Messenger will remove the implementation of OTR encryption by 30th June of 2020. 
+
+Unfortunately, it does not make sense to continue support on a rather out-dated technology, even as we see some users keep using it. For the moment, OTR (as well as OpenPGP) can be activated via the expert settings for advanced users. Pix-Art Messenger always tries to be usable for even [non-technical users](https://github.com/kriztan/Pix-Art-Messenger/issues/227), OTR however is not counted as appropriate for this.
+
+Please consider to use OMEMO in the future, many other clients has implemented this, have a look at [omemo.top](https://omemo.top/). However, if you really need to continue using it, please refer to e.g. [Miranda](https://www.miranda-ng.org/de/), [Pidgin](https://pidgin.im/), [Profanity](https://profanity-im.github.io/). You are also able to fork Pix-Art Messenger and continue the implementation on your own.
+
+Please consider the differences between encryption protocols and also advantages of OMEMO: 
+
+<img src="https://github.com/kriztan/Pix-Art-Messenger/blob/master/art/Feature_OMEMO.png" width="400">
+Source: [https://conversations.im/omemo](https://conversations.im/omemo)
+
+Some limitations of OTR: [wikipedia.org/wiki/Off-the-Record](https://en.wikipedia.org/wiki/Off-the-Record_Messaging#Limitations)
+
+Please inform your contacts who may also use it.
+
+Mastodon: https://social.tchncs.de/@pixart