CONTRIBUTING: break down steps and add an autogen.sh step. [skip ci]

Split the configure and build and test step up; they all should be done
after the user makes their changes, so add that as a step. (The autogen
and configure steps will probably only need to be done once, but the
build and test steps may have to be done repeatedly until everything
builds and works.)

Author: guyharris

CONTRIBUTING.md

@@ -61,12 +61,24 @@ and ask!
      supports these options,
    * have the `Makefile` support `make depend` and the `configure` script run it.
 
-6) Configure and build
+6) Make your changes
+
+7) Generate the configure scripts
+   ```
+   ./autogen.sh
+   ```
+
+8) Configure
+   ```
+   ./configure
+   ```
+
+9) Build
    ```
-   ./configure && make -s && make check
+   make -s
    ```
 
-7) Add/update tests
+10) Add/update tests
    The `tests` directory contains regression tests of the dissection of captured
    packets.  Those captured packets were saved running tcpdump with option
    `-w sample.pcap`.  Additional options, such as `-n`, are used to create relevant
@@ -96,22 +108,22 @@ and ask!
    It is often useful to have test outputs with different verbosity levels
    (none, `-v`, `-vv`, `-vvv`, etc.) depending on the code.
 
-8) Test using `make check` (current build options) and `./build_matrix.sh`
+11) Test using `make check` (current build options) and `./build_matrix.sh`
    (a multitude of build options, build systems and compilers). If you can,
    test on more than one operating system. Don't send a pull request until
    all tests pass.
 
-9) Try to rebase your commits to keep the history simple.
+12) Try to rebase your commits to keep the history simple.
    ```
    git fetch upstream
    git rebase upstream/master
    ```
    (If the rebase fails and you cannot resolve, issue `git rebase --abort`
    and ask for help in the pull request comment.)
 
-10) Once 100% happy, put your work into your forked repository using `git push`.
+13) Once 100% happy, put your work into your forked repository using `git push`.
 
-11) [Initiate and send](https://help.github.com/articles/using-pull-requests/)
+14) [Initiate and send](https://help.github.com/articles/using-pull-requests/)
     a pull request.
     This will trigger the upstream repository CI tests.