Documentation: Add HIG document about writing UI text

This first cut only deals with capitalization.

Documentation/HumanInterfaceGuidelines/Text.md

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+# Guidelines for user interface text in SerenityOS
+
+## Capitalization
+
+SerenityOS employs two capitalization styles:
+
+- Book title capitalization
+- Sentence-style capitalization
+
+### Book title capitalization
+
+In this style, we capitalize the first letter of the first and last word,
+as well as all words in between, *except* articles (a, an, the);
+the seven coordinating conjunctions (for, and, nor, but, or, yet, so);
+and prepositions with up to four letters (at, by, for, with, into, ...)
+
+#### Examples:
+- Create New Layer
+- Copy URL
+- Move to Front
+- Save and Exit
+- Sort by Name
+
+#### Used for:
+
+- Button text
+- Icon labels
+- Menu names
+- Menu items
+- Tab titles
+- Window titles
+- Tooltips
+
+### Sentence-style capitalization
+
+This style follows basic English sentence capitalization.
+We capitalize the first letter of the first word, along with the first letter
+of proper names, weekdays, etc.
+
+#### Examples:
+- An error occurred
+- Use system defaults
+- Copy the selected text
+- Enable Linux compatibility hacks
+
+#### Used for:
+
+- Check box labels
+- Group box labels
+- List items
+- Messages (as in message boxes)
+- Radio button labels
+- Status bar text
+- Text box labels