Extracts linux readme directions

This makes the linux directions look more like the macOS directions for the top level readme.

README.linux.md

+1. ORCA requires [CMake](https://cmake.org) and
+   [Ninja](https://ninja-build.org/), make sure you have them installed.
+   Installation instructions vary, please check the CMake and Ninja websites.
+   
+1. Install needed python modules
+
+  Add the following Python modules (2.7 & 2.6 are supported)
+
+  * psutil
+  * lockfile (>= 0.9.1)
+  * paramiko
+  * setuptools
+
+  If necessary, upgrade modules using "pip install --upgrade".
+  pip should be at least version 7.x.x.
+
+2. Verify that you can ssh to your machine name without a password
+```
+ssh <hostname of your machine>  # e.g., ssh briarwood
+```
+

README.md

@@ -36,23 +36,7 @@ to the segments, and collects the results.
 Follow [these macOS steps](README.macOS.md) for getting your system ready for GPDB
 
 ### Installing dependencies (for Linux developers)
-
-1. Install needed python modules
-
-  Add the following Python modules (2.7 & 2.6 are supported)
-
-  * psutil
-  * lockfile (>= 0.9.1)
-  * paramiko
-  * setuptools
-
-  If necessary, upgrade modules using "pip install --upgrade".
-  pip should be at least version 7.x.x.
-
-2. Verify that you can ssh to your machine name without a password
-```
-ssh <hostname of your machine>  # e.g., ssh briarwood
-```
+Follow [these linux steps](README.linux.md) for getting your system ready for GPDB
 
 <a name="buildOrca"></a>
 ### Build the optimizer
@@ -76,10 +60,6 @@ building, see the README at the following repositories:
     cd ../..
     ```
 
-1. ORCA requires [CMake](https://cmake.org) and
-   [Ninja](https://ninja-build.org/), make sure you have them installed.
-   Installation instructions vary, please check the CMake and Ninja websites.
-
 1. Install ORCA, the query optimizer:
 
     ```