Update bibtexkeypatterns.md with truncateN

Documentation for [JabRef issue 3915](JabRef/jabref#3915).

en/setup/bibtexkeypatterns.md

@@ -89,6 +89,7 @@ Generally, modifiers are applied in the order they are specified. In the followi
 * **:upper**: Forces the text inserted by the field marker to be in uppercase. For example, **\[auth:upper\]** expands the last name of the first author in uppercase.
 * **:capitalize**: Changes the first character of each word to uppercase, all other characters are converted to lowercase. For example, `an example title` will be converted to `An Example Title`
 * **:titlecase**: Changes the first character of all normal words to uppercase, all function words \(see above\) are converted to lowercase. Example: `example title with An function Word` will be converted to `Example Title with an Function Word`
+* **:truncateN**: Truncates the string after the N:th character and trims any trailing whitespaces. For example, **\[fulltitle:truncate3\]** will be convert `A Title` to `A T`.
 * **:sentencecase**: Changes the first character of the first word to uppercase, all remaining words are converted to lowercase. Example: `an Example Title` will be converted to `An example title`
 * **:\(x\)**: The string between the parentheses will be inserted if the field marker preceding this modifier resolves to an empty value. The placeholder `x` may be any string. For instance, the marker **\[volume:\(unknown\)\]** will return the entry's volume if set, and the string **unknown** if the entry's `volume` field is not set.