Added CONTRIBUTING.md (fixes issue Deducteam#42).

CONTRIBUTING.md

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+Contributing to Lambdapi
+========================
+
+Here are a few guidelines for contributions.
+
+
+General style and indentation
+-----------------------------
+
+In the interest of code source uniformity, we ask that:
+ - the *tabulation character should be banned*,
+ - one indentation unit is equal to two spaces,
+ - there should be *no trailing spaces* at the end of lines,
+ - lines length should be limited to *78 characters* (excluding newline).
+
+
+Type annotations and interface
+------------------------------
+
+Lambdapi does not use interface (or `.mli`) files. However, every function
+should be documented with its type, and an `ocamldoc` comment. We ask that
+the type of function is given with the following syntax (except for parsers
+defined in `src/parser.ml`).
+```ocaml
+(** [a_function x y z] does some things with [x], [y] and [z]. *)
+let a_function : int -> (bool -> bool) -> string -> unit = fun x y z ->
+  ...
+```
+
+Note that `ocamldoc` comments should take up as much space as possible,
+without exceeding the maximum line width. Starting at their second line
+(if any), the begining of the lines should be aligned with the first word
+of the first line.
+
+
+Other conventions
+-----------------
+
+Pattern matching arrows should be aligned to improve readability, as in the
+following.
+```ocaml
+match ... with
+| A(x)   -> ...
+| B(x,y) -> ...
+```
+
+When the action of a patter does not fit on one line, it sould be indented
+by two unite (that is, four characters).
+```OCaml
+| A(x) ->
+>>>>let y = ... in
+>>>>...
+```