Add CONTRIBUTING.md for contributing guidelines

Signed-off-by: Cloudwalk <cloudwalk@icculus.org>

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+# DarkPlaces Contributing Guidelines
+-------------------------------------------------------------------------------
+
+1. ### Do not break Quake or its mods, and any other game using the engine.
+
+   The engine has known compatibility issues with Quake and many community
+   mods. All code must not make the situation worse. This is analogous to the policy
+   of the Linux kernel to not break userspace.
+
+2. ### Sign off all of your commits if they are to be included upstream.
+
+   You must use a valid, permanent email address.
+
+2. ### All code submitted should follow the Allman style for the most part.
+
+       1. In statements, the curly brace should be placed on the next line at the
+          same indentation level as the statement. If the statement only involves
+          a single line, do not use curly braces.
+
+               ```c
+               // Example:
+               if(foo == 1)
+               {
+                       Do_Something();
+                       Do_Something_Else();
+               }
+               else
+                       Do_Something_Else_Else();
+
+               if(bar == 1)
+                       Do_Something_Else_Else_Else();
+               ```
+
+       2. Use tabs for indentation.
+
+          Use spaces subsequently for alignment of the
+          parameters of multi-line function calls or declarations, or statements.
+
+               ```c
+               // Example:
+               if(very_long_statement &&
+                  very_long_statement_aligned &&
+                  foo)
+               {
+                       if(indented)
+                       {
+                               Do_Something();
+                               Do_Something_Else();
+                       }
+               }
+               ```
+
+       3. If possible, try to keep individual lines of code less than 100
+          characters.
+
+       4. Pointer operators should be placed on the right-hand side.
+
+               ```c
+               int foo = 1;
+               int *bar = &foo;
+               ```
+
+       5. Type casts should have a space between the type and the pointer.
+
+               ```c
+               int *foo = (int *)malloc(5);
+               ```
+
+       6. Use spaces after each comma when listing parameters or variables of a
+          struct.
+
+       7. Multi-line comments should be formatted like so:
+
+               ```c
+               /*
+                * This is a multi-line comment.
+                * Sometimes, I dream about cheese.
+                */
+               ```
+
+          But this is okay too:
+
+               ```c
+               /* This is another multi-line comment.
+                * Hiya! How are you?
+                */
+               ```
+
+       8. If following these guidelines in any manner would make any code less
+          readable or harder to follow, ***ignore them***. This section is only
+          guidelines, not rules. We're not gonna beat you over the head in pull
+          requests because you misplaced a dereference operator.
+
+          For example, in some situations, placing the block on the same line as
+          the condition would be okay because it looks cleaner:
+
+               ```c
+               if (foo)  Do_Something();
+               if (bar)  Do_Something_Else();
+               if (far)  near();
+               if (boo)  AHH("!!!\n");
+               ```
+
+4. DarkPlaces is written in a special subset of C and C++ that sits in the
+   center of the Venn diagram of compatibility between the two languages.
+   While there is no practical reason for keeping it this way (yet), it is
+   generally preferred that all code submitted at least compiles under gcc and
+   g++ and clang(++). This could be done by setting the CC environment
+   variable to g++ or clang++ on the command-line before building.
+
+   Most of the differences are enforced by a compile warning set to be an
+   error (`-Werror=c++-compat`) but some are subtle and would cause behavior
+   differences between the two compilers, or are not caught by that warning.
+   The things to look out for are:
+
+       1. Character constants are `int`-sized in C but `char`-sized in C++. This
+       means `sizeof('a')` will be 4 in C, but 1 in C++.
+
+       2. `goto label;` cannot jump over a variable initialization. This will
+       cause a compiler error as C++ but is not caught by `-Wc++-compat`.