Add styleguide

STYLEGUIDE.md

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+# Style guide
+
+This is style guide for fish contributors. You should use it for any new code
+that you would add to this project and try to format existing code to use this
+style.
+
+## Formatting
+
+1. Always use 2 spaces instead tabs as indent (unless needed like `Makefile`).
+2. Opening curly bracket is always the same line as declaration.
+
+        // ✔:
+        struct name {
+          // code
+        };
+
+        void func() {
+          // code
+        }
+
+        if (...) {
+          // code
+        }
+
+        // ✗:
+        void func()
+        {
+          // code
+        }
+
+3. Put space after `if`, `while` and `for` before conditions.
+
+        // ✔:
+        if () {}
+
+        // ✗:
+        if() {}
+
+4. Put spaces before and after operators excluding increment and decrement;
+
+        // ✔:
+        int a = 1 + 2 * 3;
+        a++;
+
+        // ✗:
+        int a=1+2*3;
+        a ++;
+
+5. Never put spaces between function name and parameters list.
+
+        // ✔:
+        func(args);
+
+        // ✗:
+        func (args);
+
+6. Never put spaces after `(` and before `)`.
+7. Always put space after comma and semicolon.
+
+        // ✔:
+        func(arg1, arg2);
+
+        for (int i = 0; i < LENGTH; i++) {}
+
+        // ✗:
+        func(arg1,arg2);
+
+        for (int i = 0;i<LENGTH;i++) {}
+
+## Documentation
+
+Document your code using [Doxygen][dox].
+
+1. Documentation comment should use double star notation or tipple slash:
+
+        // ✔:
+        /// Some var
+        int var;
+
+        /**
+         * Some func
+         */
+        void func();
+
+2. Use slash as tag mark:
+
+        // ✔:
+
+        /**
+         * \param a an integer argument.
+         * \param s a constant character pointer.
+         * \return The results
+         */
+        int foo(int a, const char *s);
+
+## Naming
+
+All names in code should be `small_snake_case`. No Hungary notation is used.
+Classes and structs names should be followed by `_t`.
+
+[dox]: http://www.stack.nl/~dimitri/doxygen/ "Doxygen homepage"