Clarify SQLCIPHER_CFLAGS is an optional build environment variable

Author: developernotes

README.md

@@ -143,10 +143,11 @@ In order to build `android-database-sqlcipher` from source you will need both th
 SQLCIPHER_ROOT=/some/path/to/sqlcipher-folder \
 OPENSSL_ROOT=/some/path/to/openssl-folder \
 SQLCIPHER_ANDROID_VERSION="4.4.3" \
-SQLCIPHER_CFLAGS="-DSQLITE_HAS_CODEC" \
 make build-release
 ```
 
+You may also optionally include `SQLCIPHER_CFLAGS` to override the default features SQLCipher core is compiled with. If `SQLCIPHER_CFLAGS` is defined, make sure `-DSQLITE_HAS_CODEC` is included in the list of flags.
+
 ### License
 
 The Android support libraries are licensed under Apache 2.0, in line with the Android OS code on which they are based. The SQLCipher code itself is licensed under a BSD-style license from Zetetic LLC. Finally, the original SQLite code itself is in the public domain.