Add CONTRIBUTING.md.

Author: fukamachi

CONTRIBUTING.md

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+# Contributing to Mito
+
+Mito is an open source project and we appreciate your help!
+
+## Reporting bugs
+
+Please [open an issue](https://github.com/fukamachi/mito/issues/new) at GitHub.
+
+Good reports must include these informations:
+
+- The full backtrace with your error
+- Minimum steps to reproduce it
+- Names and versions you are using: OS, Common Lisp implementation, RDBMS (MySQL, PostgreSQL or SQLite3), ASDF and Quicklisp dist
+
+You can get informations about your environment by this code:
+
+```common-lisp
+(flet ((put (k v &rest vs)
+         (format t "~&~A: ~A~{ ~A~}~%" k v vs)))
+  (put "Machine" (software-type) (software-version))
+  (put "Lisp" (lisp-implementation-type) (lisp-implementation-version)
+       #+(and sbcl (not sb-thread)) "(without threads)")
+  (put "ASDF" (asdf:asdf-version))
+  (let ((qlversion (ql:dist-version "quicklisp")))
+    (put "Quicklisp" qlversion
+         (if (string= (car (first (ql:available-dist-versions "quicklisp")))
+                      qlversion)
+             "(latest)"
+             "(update available)"))))
+;-> Machine: Darwin 15.2.0
+;   Lisp: SBCL 1.3.1
+;   ASDF: 3.1.5
+;   Quicklisp: 2015-10-31 (latest)
+;=> NIL
+```