Update CONTRIBUTING.md

Author: vJechsmayr

CONTRIBUTING.md

@@ -4,10 +4,16 @@
 Please make sure your file is in the `LeetCode`-Folder and Named like this:
 `0001_TwoSum.py` -> 4-digit Number of the LeetCode Issue, Underscore, LeetCodeName
 
-## Pull Requests
+### Opening Issues
+When you open an issue, please make sure the Problem is not already implemented (Look at Code/Leetcode for the Problemnumber). 
+Opened Issues by existing Problem will be closed & PR made to this Issue marked as **spam**
+The Contributer who opened an issue will be assigned prefered to the issue. If there is no PR within about 7 Days the issue will be assigned to another Contributer.
+
+### Pull Requests
 Only Pull Requests **joined with an Issue** and matching the **naming-conventions** (See Folders and Files) will be merged!
 If there is no Issue joined in the PR your PR will be labeld as **spam** and closed.
 If your code don't passes the Check on LeetCode.com your PR will be labeld as **"invalid"** and the Issue stays open for the next PR!
+If your PR doesn' follow the Contributing Guidelines of this Repository it will be also marked as **spam** and closed!
 
 ## Which PR will be accepted?
 * Ones you are assigned to