readme: add an example for travis

Author: ordian

README.md

@@ -26,4 +26,12 @@ Contributions to this library are welcome. A few guidelines:
 
 The build dependency `cc` might require a more recent version of the Rust compiler.
 To ensure compilation with Rust 1.22.0, pin its version in your `Cargo.lock`
-with `cargo update -p cc --precise 1.0.41`.
+with `cargo update -p cc --precise 1.0.41`. If you're using `secp256k1` in a library,
+to make sure it compiles in CI, you'll need to generate a lockfile first.
+Example for Travis CI:
+```yml
+before_script:
+  - if [ "$TRAVIS_RUST_VERSION" == "1.22.0" ]; then
+    cargo generate-lockfile --verbose && cargo update -p cc --precise "1.0.41" --verbose;
+    fi
+```

secp256k1-sys/README.md

@@ -26,6 +26,7 @@ $ ./vendor-libsecp.sh depend <version-code> <rev>
 - Where `<version-code>` is the secp256k1-sys version number underscored: `0_1_2`.
 - Where `<rev>` is the git revision of libsecp256k1 to checkout.
 
+
 ## Linking to external symbols
 
 For the more exotic use cases, this crate can be used with existing libsecp256k1