Add package documentation

doc.go

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+// Package translitbg is a Bulgarian-language transliteration package. The API
+// can be used in a very simple way to encode cyrillic-characters text to its
+// corresponding latin-characters version.
+//
+// Example:
+//
+//	tr := translitbg.New()
+//	tr.Encode("абвгдежзийклмнопрстуфхцчшщъьюя")
+//	// Output: abvgdezhziyklmnoprstufhtschshshtayyuya
+//
+// There are no state-related parameters stored in the translitbg object, so
+// the same instance can be used to encode more text without the need to create
+// new objects every time.
+package translitbg