Add command to install a binary software in bin

Guides/shell.md

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+# Shell
+
+Useful common Unix commands and tricks
+
+## Link a binary in your local bin
+
+```sh
+ln -s -T ~/Software/GitAhead/GitAhead ~/.bin/gitahead
+```
+
+To use a physical link instead of a symlink, use `-S` instead of `-s`