README.md: document installation

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 ```
 
 
+Installation
+------------
+
+OsmoS1GW is built for common versions of Debian, Ubuntu and other distributions as a part of
+the [Osmocom binary packages](https://osmocom.org/projects/cellular-infrastructure/wiki/Binary_Packages).
+If you're not doing development, it is suggested to simply use those binary packages, rather
+than building from source.  Otherwise, you can invoke the `install` target:
+
+```
+$ sudo make install
+```
+
+This will install the following:
+
+* `/usr/lib/osmo-s1gw` - complete OTP release package
+* `/usr/bin/osmo-s1gw` - convenience script for running `osmo-s1gw`
+* `/lib/systemd/system/osmo-s1gw.service` - systemd unit file
+* `/etc/osmocom/osmo-s1gw.config` - the configuration file
+
+The installation paths can be adjusted using `DESTDIR`, `LIBDIR`, `BINDIR`,
+`CONFDIR`, and `SYSTEMDUNITDIR` variables.
+
+
 Configuration
 -------------
 

