Add example for compiling exqlite with system installation (#62)

Author: ConnorRigby

README.md

@@ -43,6 +43,32 @@ config :my_app, MyApp.Repo,
   database: "path/to/my/database.db",
 ```
 
+## Database Encryption
+
+As of version 0.9, `exqlite` supports loading database engines at runtime rather than compiling `sqlite3.c` itself.
+This can be used to support database level encryption via alternate engines such as [SQLCipher](https://www.zetetic.net/sqlcipher/design)
+or the [Official SEE extension](https://www.sqlite.org/see/doc/trunk/www/readme.wiki). Once you have either of those projects installed
+on your system, use the following environment variables during compilation:
+
+```bash
+# tell exqlite that we wish to use some other sqlite installation. this will prevent sqlite3.c and friends from compiling
+export EXQLITE_USE_SYSTEM=1
+
+# Tell exqlite where to find the `sqlite3.h` file
+export EXQLITE_SYSTEM_CFLAGS=-I/usr/local/include/sqlcipher
+
+# tell exqlite which sqlite implementation to use
+export EXQLITE_SYSTEM_LDFLAGS=-L/usr/local/lib -lsqlcipher
+```
+
+Once you have `exqlite` configured, you can use the `:key` option in the database config to enable encryption:
+
+```elixir
+config :my_app, MyApp.Repo,
+  database: "path/to/my/encrypted-database.db",
+  key: "super-secret'
+```
+
 ## Benchmarks
 
 We have some benchmarks comparing it against the `MySQL` and `Postgres` adapters.