update docs

doc/build.md

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+Install `Go >= 1.14` and `gcc`. Get the source code:
+
+    git clone https://github.com/nkanaev/yarr.git
+    git clone https://github.com/nkanaev/gofeed.git yarr/gofeed
+
+Then run one of the corresponding commands:
+
+    # create a binary for the host os
+    make build_macos    # -> _output/macos/yarr.app
+    make build_linux    # -> _output/linux/yarr
+    make build_windows  # -> _output/windows/yarr.exe
+
+    # ... or start a dev server locally
+    go run main.go      # starts a server at http://localhost:7070
+
+    # ... or build a docker image
+    docker build -t yarr .