Add electrodinàmica resum

Add a summary of the special relativity chapter.

Change-Id: I1d002335b853f2bccb15b080683dac4a2990330e
diff --git a/quad9/electrodinamica/resum/.gitignore b/quad9/electrodinamica/resum/.gitignore
new file mode 100644
index 0000000..120e611
--- /dev/null
+++ b/quad9/electrodinamica/resum/.gitignore
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+*.pdf
+figures/