Add lecture notes on String

[aubertc]

Lecture notes on string are missing as of now. They should discuss:

• How to create a string from an array of char, or using new string('*', 10)
• The difference between empty string and null strings,
• Possibly, the @ "trick",
• How to go from string to array of characters, using ToCharArray() and [1] (with a comment on length & null values)
• Why we can compare strings using == while we can't compare references using this operator in general,
• The SubString method (with 1 or 2 arguments), the Split method, the Join() method (and other representative methods?),