[Analysis] Add more notes

semester3/analysis-ii/cheat-sheet-jh/parts/vectors/differentiation/02_differential.tex

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 \subsection{The differential}
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 \compactdef{Differentiable function} We have function $f: X \rightarrow \R^m$, linear map $u : \R^n \rightarrow \R^m$ and $x_0 \in X$. $f$ is differentiable at $x_0$ with differential $u$ if
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 We denote $\dx f(x_0) = u$.
 If $f$ is differentiable at every $x_0 \in X$, then $f$ is differentiable on $X$.
 
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