DOC: Fix typos in README and CONTRIBUTING

Few typos detected by misspell in the README and CONTRIBUTING.
Even if one of them is on a listing of commits. I'm assuming that
if we want to enforce less typos in the commits, having one in the
contributing guide is not the best example.
diff --git a/README b/README
index fb3b945..4c487a6 100644
--- a/README
+++ b/README
@@ -116,7 +116,7 @@
 currently supported branches (0.9.8, 1.0.0, 1.0.1, 1.0.2 and 1.1.0 at the time
 of writing). Branch 1.0.2 is currently recommended for the best combination of
 features and stability. Asynchronous engines require OpenSSL 1.1.0 though. It's
-worth mentionning that some OpenSSL derivatives are also reported to work but
+worth mentioning that some OpenSSL derivatives are also reported to work but
 may occasionally break. Patches to fix them are welcome but please read the
 CONTRIBUTING file first.
 
@@ -189,7 +189,7 @@
       silently fail. Pthreads are enabled using USE_PTHREAD_PSHARED=1.
 
   Synchronization operations :
-    - internal spinlock : this mode is OS-independant, light but will not
+    - internal spinlock : this mode is OS-independent, light but will not
       scale well to many processes. However, accesses to the session cache
       are rare enough that this mode could certainly always be used. This
       is the default mode.