Add semver.

semver.md

@@ -0,0 +1,56 @@
+---
+title: Semver
+layout: default
+---
+
+Given a version number `MAJOR.MINOR.PATCH`:
+
+ * `MAJOR` = incompatible API changes.
+ * `MINOR` = add functionality. (backwards-compatible)
+ * `PATCH` = bug fixes. (backwards-compatible)
+
+### Simple ranges
+
+      1.2.3
+     =1.2.3
+     >1.2.3
+     <1.2.3
+    >=1.2.3
+
+Note that suffixed versions (`1.2.3-rc1`) are not matched.
+
+### Ranges
+
+    ~1.2.3    : is >=1.2.3 <1.3.0
+
+    ^1.2.3    : is >=1.2.3 <2.0.0
+    ^0.2.3    : is >=0.2.3 <0.3.0   (0.x.x is special)
+    ^0.0.1    : is  =0.0.1          (0.0.x is special)
+
+    ^1.2      : is >=1.2.0 <2.0.0   (like ^1.2.0)
+    ~1.2      : is >=1.2.0 <1.3.0   (like ~1.2.0)
+
+    ^1        : is >=1.0.0 <2.0.0
+    ~1        : same
+     1.x      : same
+     1.*      : same
+     1        : same
+
+    *         : any version
+    x         : same
+
+### Pre-releases
+
+    1.2.3-prerelease+build
+
+### Explanation
+
+ * `^` means "compatible with"
+ * `~` means "reasonably close to"
+ * `0.x.x` is for "initial development"
+ * `1.x.x` means public API is defined
+
+### References
+
+ * http://semver.org/
+ * https://www.npmjs.org/doc/misc/semver.html