Update doc "how to build from source" (#111)

Author: kezhenxu94

CONTRIBUTING.md

@@ -20,6 +20,10 @@ Most of the contributions that we receive are code contributions, but you can al
 * If you're unable to find an issue addressing the problem, [open a new one](https://github.com/apache/skywalking/issues/new).
 Be sure to include a **title and clear description**, as much relevant information as possible, and a **code sample** or an **executable test case** demonstrating the expected behavior that is not happening.
 
+## Compiling and building
+
+Check [here](docs/FAQ.md#q-how-to-build-from-sources).
+
 ## Add a new feature or enhance an existing one
 
 _Before making any significant changes, please [open an issue](https://github.com/apache/skywalking/issues)._

docs/FAQ.md

@@ -14,7 +14,15 @@ you can also disable the plugins via environment variables `SW_AGENT_DISABLE_PLU
 
 #### Q: How to build from sources?
 
-#### A: If you want to build the SkyWalking Python Agent from source codes, for some reasons, for example, you want to try the latest features that're not released yet, please clone the source codes from GitHub and `make install` it:
+#### A: Download the source tar from [the official website](http://skywalking.apache.org/downloads/), and run the following commands to build from source
+
+```shell
+$ tar -zxf skywalking-python-src-<version>.tgz
+$ cd skywalking-python-src-<version>
+$ make install
+```
+
+If you want to build from the latest source codes from GitHub, for some reasons, for example, you want to try the latest features that are not released yet, please clone the source codes from GitHub and `make install` it:
 
 ```shell
 git clone https://github.com/apache/skywalking-python
@@ -23,4 +31,4 @@ git submodule update --init
 make install
 ``` 
 
-**NOTE** that because we use Git submodule, the aforementioned commands don't apply if you download the source codes (`.tgz`) from our official website.
+**NOTE** that only releases from [the website](http://skywalking.apache.org/downloads/) are official Apache releases.