Updated templates, added pull request template.

.github/issue_template.md

@@ -1,7 +1,12 @@
-(Please carefully read guidelines in [CONTRIBUTING.md](https://github.com/ocornut/imgui/blob/master/.github/CONTRIBUTING.md) then delete this line)
+You may use the Issue tracker to ask for help, submit bug reports, feature requests or suggestions. 
+Please carefully read this document before doing so:
+[CONTRIBUTING.md](https://github.com/ocornut/imgui/blob/master/.github/CONTRIBUTING.md).
 
-You can include code this way:
+You can include code snippets using `Begin()` for short in-line snippets, or:
 ```cpp
 ImGui::Begin("Hello");
 ImGui::ThisIsMoreCode();
 ```
+For multiline snippets.
+
+(Clear this form before submitting your issue)