Added rule on duplicating same tools in many categories

Contribution_Guidelines.md

@@ -7,11 +7,17 @@ If you would like to contribute, just send a Pull Request. Please:
 * End all descriptions with a full stop/period and remove trailing whitespace.
 * Check spelling and grammar.
 
+## One tool can go only to one category
+Every category presents some design work or task. Many design tools allow us to solve multiple tasks. But in this list, one tool with full description & link is always submitted only to one category (section). Why do so? Because we want to avoid many duplications and misunderstanding around tool functionality.
+
+However, if the tool can be used for another task, just mention tool name in the additional sentence at the end of another category, like here: `You can also do prototyping with Figma, Adobe XD, Sketch and InVision Studio, which are mentioned in the UI Design Tools section.`
+
+
 ## Link Format
-* Please, use the following format for the links in Pull Requests: ``[Tool](link) — description``.
+* Please, use the following format for the links in Pull Requests: ``[Tool](link) — description``. The description should start from the lower-case letter unless it's starting with the tool name.
 
 * Please, add the appropriate labels:
-  * ![free.svg](https://github.com/LisaDziuba/Awesome-Design-Tools/blob/master/Media/free.svg) for free tools (entirely free tools, not freemium). Just copy & paste the following snippet: ``![free.svg](https://github.com/LisaDziuba/Awesome-Design-Tools/blob/master/Media/free.svg)``
+  * ![free.svg](https://github.com/LisaDziuba/Awesome-Design-Tools/blob/master/Media/free.svg) for free tools. **Free means** entirely free tools, not freemium or those which have free trial, free limited functionality. For adding label just copy & paste the following snippet: ``![free.svg](https://github.com/LisaDziuba/Awesome-Design-Tools/blob/master/Media/free.svg)``
   * ![open-source.svg](https://github.com/LisaDziuba/Awesome-Design-Tools/blob/master/Media/open-source.svg) for open-source tools. Use the following snippet: ``![open-source.svg](https://github.com/LisaDziuba/Awesome-Design-Tools/blob/master/Media/open-source.svg)``
   * ![mac.svg](https://github.com/LisaDziuba/Awesome-Design-Tools/blob/master/Media/mac.svg) for Mac only tools. Use the following snippet: ``![mac.svg](https://github.com/LisaDziuba/Awesome-Design-Tools/blob/master/Media/mac.svg)``