Added instructions for contributing to pg

CONTRIBUTING.txt

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+Contributions to pyqtgraph are welcome! 
+
+Please use the following guidelines when preparing changes:
+
+* The preferred method for submitting changes is by github pull request. If
+  this is inconvenient, don't hesitate to submit by other means.
+
+* Pull requests should include only a focused and related set of changes. 
+  Mixed features and unrelated changes (such as .gitignore) will usually be
+  rejected.
+
+* For major changes, it is recommended to discuss your plans on the mailing 
+  list or in a github issue before putting in too much effort.
+
+      * Along these lines, please note that pyqtgraph.opengl will be deprecated 
+        soon and replaced with VisPy.
+
+* Writing proper documentation and unit tests is highly encouraged. PyQtGraph
+  uses nose / py.test style testing, so tests should usually be included in a
+  tests/ directory adjacent to the relevant code. 
+
+* Documentation is generated with sphinx; please check that docstring changes
+  compile correctly.
+
+* Style guidelines:
+
+      * PyQtGraph prefers PEP8 for most style issues, but this is not enforced
+        rigorously as long as the code is clean and readable.
+
+      * Exception 1: All variable names should use camelCase rather than 
+        underscore_separation. This is done for consistency with Qt
+
+      * Exception 2: Function docstrings use ReStructuredText tables for 
+        describing arguments:
+
+        ```
+        ============== ========================================================
+        **Arguments:**
+        argName1       (type) Description of argument
+        argName2       (type) Description of argument. Longer descriptions must
+                       be wrapped within the column guidelines defined by the
+                       "====" header and footer.
+        ============== ========================================================
+        ```
+
+        QObject subclasses that implement new signals should also describe 
+        these in a similar table.