Add PNGProfile and fill README (#134)

orx-video-profiles/src/main/kotlin/PNGProfile.kt

@@ -0,0 +1,16 @@
+package org.openrndr.extra.videoprofiles
+import org.openrndr.ffmpeg.VideoWriterProfile
+
+/**
+ * This profile requires specifying a file name like this
+ * outputFile = "frame-%05d.png"
+ * where `%05d` means "zero-padded five-digit frame number".
+ * The frame number format is not optional.
+ */
+class PNGProfile : VideoWriterProfile() {
+    override val fileExtension = "png"
+
+    override fun arguments(): Array<String> {
+        return arrayOf("-vf", "vflip")
+    }
+}