Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Coding standards
+
+* 4 spaces are used for tabulation.
+* Unix line endings are used.
+* Curlies are always explicitly written.
+
+## If
+
+```
+if (something) {
+    ...
+} else {
+    ...
+}
+```
+
+## for
+
+```
+for (uint8_t i; i<j; i++) {
+    ...
+}
+```
+
+## while
+
+```
+while (condition) {
+    ...
+}
+```
+
+## function
+
+```
+void do_this()
+{
+    ...
+}
+```