Adding a page to the compatibility site referenced by the 2.1 CDD.
This page lists the allowed version strings.

Change-Id: Iffbe0f1739a5daae8f2aa447b86af12aa7142a31
diff --git a/pdk/docs/compatibility/2.1/versions.jd b/pdk/docs/compatibility/2.1/versions.jd
new file mode 100644
index 0000000..9687a96
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+++ b/pdk/docs/compatibility/2.1/versions.jd
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+page.title=Permitted Version Strings for Android 2.1
+<p>As described in Section 3.2.2 of the <a
+href="{@docRoot}compatibility/android-2.1-cdd.pdf">Android 2.1 Compatibility
+Definition</a>, only certain strings are allowable for the system property
+<code>android.os.Build.VERSION.RELEASE</code>. The reason for this is that
+applications and web sites may rely on predictable values for this string, and
+so that end users can easily and reliably identify the version of Android
+running on their devices.</p>
+<p>Because subsequent releases of the Android software may revise this string,
+but not change any API behavior, such releases may not be accompanied by a new
+Compatibility Definition Document. This page lists the versions that are
+allowable by an Android 2.1-based system. The only permitted values for
+<code>android.os.Build.VERSION.RELEASE</code> for Android 2.1 are:</p>