Added contributing guidelines

CONTRIBUTING.md

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+# Contributing to `clang-uml`
+
+Thanks for taking interest in `clang-uml`!
+
+> Please, make sure you're ok with [Code of conduct](./CODE_OF_CONDUCT.md) and [LICENSE](./LICENSE.md)
+
+
+## If you found a bug
+
+* Optimally, fork the repository, create some branch, and add a test case reproducing the issue (it's easy!) - 
+  check what is the highest test case number in a specific category and use the test case generation script with
+  a consecutive number:
+   ```bash
+   # To generate a class diagram test case with number 50
+   ./util/generate_test_case.py class 50
+   ```
+  This will generate a new test case template in the `tests/t00050` directory. In `tests/t00050/t00050.cc` write
+  the C++ code which triggers the issue, and in `tests/t00050/test_case.h` write the test checks.
+  The test case must be also added manually to `tests/test_cases.cc`:
+   ```cpp
+   ...
+   #include "t00047/test_case.h"
+   #include "t00048/test_case.h"
+   #include "t00049/test_case.h"
+   #include "t00050/test_case.h"
+   ...
+   ```
+  
+  Finally, create an issue with a link to your branch with the new test case.
+
+* If the issue occurs with some publicly available code (e.g. available on GitHub), please create a new issue
+  and describe the steps necessary to reproduce the issue including:
+  * Link to source code and specific version used
+  * `clang-uml` version used
+  * Operating system, compiler and linked LLVM version
+  * `.clang-uml` configuration file used
+  * Exact command line used to execute `clang-uml`
+  * Excerpt from the console logs 
+
+* If the code on which `clang-uml` fails cannot be shared, please create an issue and try to provide at least:
+    * `clang-uml` version used
+    * Operating system, compiler and linked LLVM version
+    * `.clang-uml` configuration file used
+    * Exact command line used to execute `clang-uml`
+    * Excerpt from the console logs 
+    * Description of the code fragment on which the error occurs (e.g. a variadic template or anonymous struct)
+
+## If you would like to fix a bug
+* Fork the repository and create a branch
+* Create a new test case which triggers the bug (see above how to create a new test case)
+  ```bash
+  # Make sure the test case fails
+  make test
+  ```
+* Fix the code so that the test case passes
+  ```bash
+  # Again, make sure the test case fails and other still pass
+  make test
+  ```
+* Commit the changes including all new files
+* Make sure the code is properly formatted:
+  ```bash
+  # Requires docker to ensure consistent formatting through specific clang-format version
+  make format
+  git add . && git commit -m "Fixed formatting"
+  ```
+* Create a pull request from your branch to `master` branch
+
+## If you would like to add a feature
+* First, please check if the feature isn't already mentioned in the issues or existing PR's
+* If not, create a new issue and describe as good as possible the new feature including:
+  * Rationale
+  * If the feature adds new configuration options, provide an example of new configuration file
+  * If the feature adds a new diagram feature, please add an example C++ code and expected PlantUML diagram which should
+    be generated through the feature
+* To maximize the chances of accepting the new feature, wait for some discussion on the issue before implementing
+  the feature to ensure we're on the same page as to its purpose and possible implementation
+* Next, implement the feature, please try to adapt to the overall code style:
+  * 80-character line width
+  * snakes over camels
+  * use `make format` before submitting PR to ensure consistent formatting
+* Add test case (or multiple test cases), which cover the new feature
+* Finally, create a pull request!
+
+