Add documentation about using system install sqlite3 instead

Author: warmwaffles

README.md

@@ -48,6 +48,51 @@ config :exqlite, default_chunk_size: 100
   not specifying the chunk size explicitly.
 
 
+## Advanced Configuration
+
+### Using System Installed Libraries
+
+This will vary depending on the operating system.
+
+```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/include
+
+# tell exqlite which sqlite implementation to use
+export EXQLITE_SYSTEM_LDFLAGS=-L/lib -lsqlite3
+```
+
+After exporting those variables you can then invoke `mix deps.compile`. Note if you
+re-export those values, you will need to recompile the `exqlite` dependency in order to
+pickup those changes.
+
+### 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 :exqlite, key: "super-secret'
+```
+
 ## Usage
 
 The `Exqlite.Sqlite3` module usage is fairly straight forward.