Update README.md (#1241)

Include CONTRIBUTING.md guidelines as collapseable elements.
Remove white background from screenshots
Optimize app-icon.png and screens.png using `optipng -o2`

.github/ISSUE_TEMPLATE.md

@@ -1 +1,13 @@
-**Please read https://github.com/inorichi/tachiyomi/blob/master/.github/CONTRIBUTING.md before posting**
+**Please fill out this form and remove the first two lines before posting.**
+**If your issue is a request for a catalogue it belongs here https://github.com/inorichi/tachiyomi-extensions/**
+**App version:**
+
+**Issue/Request:**
+
+**Steps to reproduce (if applicable)**
+
+ 1.
+ 2.
+ 3.
+
+**Other details:**