Convention of a += operator in pseudocode
The +=et al are C shortcuts, adopted by many languages. Most anyone should understand them.
Then again, this is (pseudo)code, you certainly can make up you own variant. Just make sure to define clearly what any not-so-standard constructions mean (I'd go for += and so on, but then again, I'm a C fan...). But make sure to be consistent, i.e., use = for assignment and += for your operation, don't mix it with e.g. $\leftarrow$, and state that you'll use C notation for assignments.
Whatever you do, always keep in sight that the objective is understanding by your gentle reader, not extreme rigour or algorithmic details.