update readme with supported platforms

CONTRIBUTING.rst

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+============
+Contributing
+============
+
+Want to improve the quality of tmux-mem-cpu-load code? Great! Here's a quick
+guide:
+
+1. Fork, then clone the repo:
+
+    git clone git@github.com:your-username/tmux-mem-cpu-load
+
+2. Make your change. Add tests for your change.
+3. See if it compiles and runs like it should.
+4. Run tests to check if you didn't break anything:
+
+    make test
+
+Push to your fork and `submit a pull request`_.
+
+At this point you're waiting on us. We'll review your changes as soon as we can.
+Before merging your changes we may request you to make some changes or
+corrections.
+
+Style guidelines
+----------------
+
+You'll need to follow the subsequent rules in order to get your code merged:
+
+* Use Allman_ style for block braces.
+* No space before `(`
+* Add space after each `(` and before each `)`
+* Use braces single line statements
+* Don't use mixed case naming style, use underscores instead.
+  Bad example:
+
+    int myAwesomeVariable = 0;
+    doSomething( myAwesomeVariable );
+
+  Good example:
+
+    int my_awesome_variable = 0;
+    do_something( my_awesome_variable );
+
+* Don't vertically align tokens on consecutive lines.
+* If you break up an argument list, align the line to opening brace
+* Use 2 space indentation (no tabs)
+* Use spaces around operators, except for unary operators, such as `!`.
+* Add LICENSE header in new files you create.
+* Put vim modeline as the first line of file header
+* Use the lower-case for CMake commands
+* Do not add trailing whitespace
+
+.. _`submit a pull request`: https://github.com/thewtex/tmux-mem-cpu-load/compare/
+.. _Allman: http://en.wikipedia.org/wiki/Indent_style#Allman_style