add a pull resquest template

Author: mozman

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

@@ -0,0 +1,18 @@
+---
+name: Pull Request Template
+about: Create a pull request to merge your contribution.
+
+---
+
+Describe your changes. Please, keep the description short and simple.
+
+__Important:__ choose `base: master` as the target branch
+
+_Optional_ things you can do before requesting a review:
+
+- Type check the code: `mypy --ignore-missing-imports -p ezdxf`, `ezdxf` has to be installed in dev-mode for that.
+- Run the core tests: `pytest tests`
+- Format the code by `black` with the default settings.
+- Write tests.
+- Write a documentation, when adding new features. Doesn't have to be `RestructuredText`, 
+  `Markdown` or simple text is OK, just enough to save me time.