brew.md: add Linuxbrew version (#1229)

See https://github.com/Linuxbrew/homebrew-core/pull/1472 for context.

pages/linux/brew.md

@@ -0,0 +1,27 @@
+# brew
+
+> The Homebrew package manager for Linux.
+
+- Search formula:
+
+`brew search {{text}}`
+
+- Install formula:
+
+`brew install {{formula}}`
+
+- List installed formulae [with matching name]:
+
+`brew list {{[text]}}`
+
+- Get latest version of installed formula (passing no formula updates all installed formulae):
+
+`brew upgrade {{[formula]}}`
+
+- Update brew:
+
+`brew update`
+
+- Switch version of formula:
+
+`brew switch {{formula}} {{version}}`